You can do this a couple of ways.

Via the "Solution Explorer"

1. Simply right-click the "References" folder and select "Manage NuGet Packages..."
2. Once that window comes up click on the option labeled "Online" in the left most part of the dialog.
3. Then in the search bar in the upper right type "json.net"
4. Click "Install" and you're done.

Via the "Package Manager Console"

1. Open the console. "View" > "Other Windows" > "Package Manager Console"
2. Then type the following:

   Install-Package Newtonsoft.Json

For more info on how to use the "Package Manager Console" check out the nuget docs.

You can right click on the project where you want to install newtonsoft nuget package.

You will find an option "Manage NuGet packages".

Then search online for "newtonsoft".

This would add a packages.config file in your project (depending on which framework you are targeting to ).

this new file contains all the nuget dependencies required for this project.

Refer this blog for more details

I might be just be late to answer, but this is usually caused by the incorrect package source, different packages come from different sources, for example angularJS.core packages comes from nuget.org source so if you search it and you have Microsoft and .NET set as your source, then you wont be able to find it.

To attempt to answer your question, please select the change the source on your your PM console(see attached screenshot) , you might also want to make sure that you point to the correct project(attached screenshot will also guide you on that regard). Or even better just select "All" on your source and you should be good

Its probably something in your build system: the dll is likely getting cleaned (as in deleted by a rebuild), or is in a directory that isn't under your solution, or simply isn't getting copied to the output directory.

I'm sorry I don't have a more specific piece of advice. What I'm going to do though is suggest you use a shot-gun to kill a mosquito: use NuGet. In C# with Visual Studio (or any other .net IDE), you really want to use the nuget package manager, since it works so well.

1. Install the visual studio extension,
2. right click on your solution (dont do the project, do the solution)
3. select 'Manage Nuget Packages for Solution'
4. click 'online', in the search box type 'newtonsoft'
5. click 'Install' on 'Json.NET' (probably, what your looking for might be in another package, I'll ask you to track it down).

This will put it in a library directory, include it in the build path, and allow you to use the Intelli sense to auto-add the appropriate reference from your project.