Yeah... the Steam Mac client is not as reliable in my experience. The verification of the steam cache doesn't always get all the files actually downloaded if it missed some. You can delete I think some .vdf files in the steam common folder with the name "avww" in them, and that might help. Otherwise if you copy out your world folders and settings.dat and inputbindings.dat files (just to be safe), and then install the demo from our site into the same folder that steam was using, that would also fix it.