Mac version won't be on steam because Ambrosia has an exclusive contract to supply the mac version, or so i heard. So sorry, there is no official mac+steam version out there, and it doesn't look like it ever will.
EDIT:
AFAIK ambrosia made an exception for the humble bundle.
If you don't care about steam and just want to get the game to run on osx, take any version you have (win32/linux/humblebundle/ambrosia/whatever) and read
this.