So, it turns out that I just needed to write sudo su postgres without the -. I then entered my user password, wrote psql, entered the password and then I was able to interact with Postgres using the command line.

If you're having difficulty as I was, try this and set the path as I did in my original post.

Appears "the way" to install the client, if you want to use hombrew, is:

brew install postgresql@15

(you need to specify the version after @) then psql (the client command line) will now be available to you (it also installs a local Postgres server/database, but you don't have to use that if all you want is the client).

Apparently there's also a 'wrapper' to the psql command to make it more "user friendly" also available via homebrew (brew install pgcli) in case interesting. Pgcli is a command line interface for Postgres with auto-completion and syntax highlighting.

Another option is to install the libpq homebrew package, but it doesn't get in your PATH.

brew install libpq

So follow the instructions it says after install out to add it to your PATH, or brew link --force libpq or add a symlink.

The error message asks the right question: is the server accepting connections on port 5432? The lsof output you provided indicates that no, it is not. PostgreSQL is listening on localhost:5432, meaning it will only accept connections from the database server itself.

Open the server's postgresql.conf, set listen_addresses = '*', and restart PostgreSQL. It will then be listening for connections over all interfaces, thus accepting connections over the network.

From here, the next problem you're likely to run into is authentication. (PostgreSQL will accept connections, but you probably haven't told it what to do once it has a connection from across the network.) Ensure the server's pg_hba.conf has an entry matching your database/user/source address combination -- something like host all all 10.0.0.0/8 md5 is probably appropriate -- and reload PostgreSQL as necessary to apply changes.

The md5 part tells PostgreSQL to attempt authentication using a password, so you will also need to set a password on the user in question if you don't have one set already. From however you normally administer your database (e.g. psql template1 on the database server), say ALTER USER whomever WITH PASSWORD 'your_new_password'.