New Method using Etcher

1. Make sure you have a USB Stick with at least 4GB of free space.
2. Get your Ubuntu ISO from here.
3. Open Disk Utility from Applications > Utilities
4. Select the USB stick and select Erase (This will remove all data stored on the USB drive)
5. Select the format: MS-DOS (FAT) and scheme: GUID Partition Map

6. Click Erase

   

7. Download, install and open Etcher (By default MacOS blocks running apps from unidentified developers. Follow this tutorial if you are experiencing problems with this.)
8. Select image and navigate to the location you saved the Ubuntu ISO you have downloaded in step 2
9. Select the USB drive you have formatted in step 3-6.

10. Click Flash

    

11. Congratulations! You now have Ubuntu on a USB stick, bootable and ready to go!

Old Method using UNetbootin (might still work)

1. Make sure you have a USB Stick with at least 4GB of free space.
2. Download and install UNetbootin USB installer
3. Download The Ubuntu Desktop ISO of your choice

4. Launch UNetbootin and allow the osascript to make changes

   

5. Select the Diskimage radio button and then click the ... button

   

6. Select the ubuntu ISO file you downloaded and click 'Open'

   

7. Then select your flash drive in the bottom row and click 'OK'
8. After Unetbootin finishes, click 'Exit' and now you have yourself an Ubuntu USB stick.

Source

I would suggest using a command line utility, dd.

1. Attach the USB that you intend to use, note that this process will fully wipe it.
2. List the drives/partitions with diskutil list (find the /dev/disk#)
3. Run dd if=/Users/USERNAME/Downloads/ubuntu-13.04x64.iso of=/dev/disk# bs=1M

Once that appears to be done, type sync and make sure your drive is written to. That should be all you need, just be sure to not mistype anything - since you could kill your main drive by overwriting sectors.

Follow these steps to create an .img file from the .iso file you have, and then copy to the USB stick. This will also change the filesystem that is on the USB to make it bootable.

1. Open the Terminal Application.

2. Type command to convert the .iso file to .img using the convert option.

   hdiutil convert -format UDRW -o /path/to/target.img /path/to/ubuntu.iso

3. Insert your flash media.

4. Type command to determine the device node assigned to your flash media (e.g. /dev/disk2).

   diskutil list

5. Type command to unmount the flash (replace N with the disk number from the last command; in the previous example, N would be 2).

   diskutil unmountDisk /dev/diskN

6. Type DD command (replace /path/to/downloaded.img with the path where the image file is located; for example, ./ubuntu.img). Prepend the device path with "r" for the raw path which is much faster than without the "r".

   sudo dd if=/path/to/downloaded.img of=/dev/rdiskN bs=1m

   Note: your file might also be called downloaded.img.dmg. That's okay.

7. Type command to eject the flash drive.

   diskutil eject /dev/diskN

8. Restart your Mac and press alt/option key while the Mac is restarting to choose the USB stick as the boot disk.

Taken from here:

We would encourage Mac users to download Ubuntu Desktop Edition by burning a CD. But if you prefer to use a USB stick, please follow the instructions below.

Note: this procedure requires that you create an .img file from the .iso file you download. It will also change the filesystem that is on the USB stick to make it bootable, so backup all data before continuing.

Tip: Drag and drop a file from Finder to Terminal to 'paste' the full path without risking typing errors.

1. Download Ubuntu Desktop

2. Open the Terminal (in /Applications/Utilities/ or query Terminal in Spotlight)

3. Convert the .iso file to .img using the convert option of hdiutil. Example:

   hdiutil convert -format UDRW ~/path/to/target.iso -o ~/path/to/ubuntu.img
   

   Note: OS X tends to put the .dmg ending on the output file automatically.

4. Run diskutil list to get the current list of devices

5. Insert your flash media

6. Run diskutil list again and determine the device node assigned to your flash media
   (e.g. /dev/disk2)

7. Run

   diskutil unmountDisk /dev/diskN
   

   (replace N with the disk number from the last command; in the previous example, N would be 2)

8. Execute the following command while replacing /path/to/downloaded.img with the path where the image file is located; for example, ./ubuntu.img or ./ubuntu.dmg).

   sudo dd if=/path/to/downloaded.img of=/dev/rdiskN bs=1m
   

   • Using /dev/rdisk instead of /dev/disk may be faster.
   • If you see the error dd: Invalid number '1m', you are using GNU dd. Use the same command but replace bs=1m with bs=1M.
   • If you see the error dd:/devdiskN: Resource busy, make sure the disk is not in use. Start the Disk Utility.app and unmount (don't eject) the drive.

9. Run diskutil eject /dev/diskN and remove your flash media when the command completes

10. Restart your Mac and press Alt while the Mac is restarting to choose the USB-Stick

You will of course need a OS X Installer Image. To get this you can follow the instructions here

After you have got your installer media, you can use ubunut's built-in utility Startup Disk Creator to make a bootable USB. SO, open dash and search for Startup Disk Creator and open it. Follow the on-screen instructions or refer to this document from ubuntu.

1. You will need:

   • A 4GB or larger USB stick/flash drive
   • An Apple computer or laptop running macOS
   • An Ubuntu ISO file. See Get Ubuntu for download links

2. To ensure maximum compatibility with Apple hardware, first blank and reformat the USB stick using Apple's "Disk Utility", but this step can be skipped if you intend to use the USB stick with only generic PC hardware.

   • Launch Disk Utility from Applications -> Utilities or Spotlight search.

   • Insert your USB stick and observe the new device added to Disk Utility.

   • Select the USB stick device (you may need to enable the option View -> Show All Devices) and select Erase from the toolbar (or right-click menu).

   • Set the format to MS-DOS (FAT) and the scheme to GUID Partition Map.

   • Check you've chosen the correct device and click Erase.

     

   Warning: Disk Utility needs to be used with caution as selecting the wrong device or partition can result in data loss.

3. Install and run balenaEtcher

   To write the ISO file to the USB stick, use a free and open source application called balenaEtcher. After downloading this and clicking to mount the package, balenaEtcher can either be run in place or dragged into your Applications folder.

   By default, recent versions of macOS block the running of applications from unidentified developers. To side-step this issue, enable App Store and identified developers in the Security & Privacy pane of System Preferences. If you are still warned against running the application, click Open Anyway in the same pane.

   

4. balenaEtcher configuration

   balenaEtcher will configure and write to your USB device in three stages, each of which needs to be selected in turn:

   • Select image will open a file requester from which should navigate to and select the ISO file downloaded previously. By default, the ISO file will be in your Downloads folder.

   • Select drive, replaced by the name of your USB device if one is already attached, lets you select your target device. You will be warned if the storage space is too small for your selected ISO.

   • Flash! will activate when both the image and the drive have been selected. As with Disk Utility, balenaEtcher needs low-level access to your storage hardware and will ask for your password after selection.
              

5. Write to device

   After entering your password, balenaEtcher will start writing the ISO file to your USB device.

   The Flash stage of the process will show progress, writing speed and an estimated duration until completion. This will be followed by a validation stage that will ensure the contents of the USB device are identical to the source image.

   When everything has finished, balenaEtcher will declare the process a success.

   Congratulations! You now have Ubuntu on a USB stick, bootable and ready to go.

   

   Warning: After the write process has completed, macOS may inform you that 'The disk you inserted was not readable by this computer'. Don't select Initialize. Instead, select Eject and remove the USB device.

6. Boot your Mac

   If you want to use your USB stick with an Apple Mac, you will need to restart or power-on the Mac with the USB stick inserted while the Option/alt ⌥ key is pressed.

   This will launch Apple's 'Startup Manager' which shows bootable devices connected to the machine. Your USB stick should appear as gold/yellow and labelled 'EFI Boot'. Selecting this will lead you to the standard Ubuntu boot menu.

   

Source: Create a bootable USB stick on macOS