the good news is that I can apply the plugin by using the absolute path: "/Applications/calibre.app/Contents/MacOS/calibre-customize -b ." in my directory. That works fine and applies the plugin.
The other issue I am having is when developing the library. My text editor doesn't know that calibre exists so when I write:
from calibre.customize import FileTypePlugin"
I don't get any library hints. Do I need to `pip` install a calibre library to get those hints?