As far as I know, there's no way of running Steam on a Macintosh.
There was supposedly a Mac port designed by Valve long ago (circa 1999), but it was killed off because Valve didn't want to keep the two versions synced for networking.
There's (legal) emulators of the Windows API for Linux, but I don't know what (if any) options you have for the Mac in that area. That's a whole different barrel of monkeys.