Actually this DOES work on steam and on the Mac version. You are just looking in the wrong place.
The game files are hidden in the users library folder, which is hidden.
so look in
/Users/<your username>/Library/Applications
You won't be able to navigate there using the Finder, you have to use the GO command (shift Command G).
Once you ar there you will see the Avadon folder and everything works as originally described.
I've tested it and have it working on Mac right now purchased through steam.