...well, unfortunately I'm not really a user on Mac's. But I remembered that I have a (very old) Mac Mini "lying around" somewhere. And since I was curious I reactivated it (hadn't started that old thing for years now...). And it worked! The Mac is running, the game is running, and even my Mod is running on it, too!
My steps for that:
One problem: I'm a German speaking user, so my whole system is in German. So maybe my translations from the Mac are not correct, sorry for that.
- install the base game
- download my Mod
- extract my Zip file temporarily and open the extracted folder
- now open a second "Finder" and go to your main game app/program file (where you normaly start the game). Right click on it. There should be an option like "package contents" or something similar, in my system it was in the upper part of the context menu. Click on it.
- now open: Contents -> Resources -> autorun -> game
- switch back to your first "Finder" with my Mod files and drag and drop all of them (and the folder "music" when using the Mod with music) into the already open .../game folder of the second "Finder"
- on my Mac I was asked for my password then, don't know if that's the same for you, too
That's it. Now you can (hopefully) start the game with my installed Mod