Hi Jeremiah,
The last time I used Lilypond on the Mac, I used it via a GUI.
Looks like I have to do something different here. I am comfortable
running it from Terminal, if that is what I have to do. I've never
done that. Where do I find instructions for using the package I just
downloaded?
Most people use Frescobaldi for editing LilyPond files:
https://frescobaldi.org/download I hope it works on your MacOS version
(but I think it should).
Thankfully, LilyPond's interface is stable enough so you do not need to
have a Frescobaldi version that knows about the recent infrastructure
changes (Guile 2, new release mode etc.)
Lukas