If you go to your game section on this site (click the little joystick in the top right corner) and it'll show you what games you own and the links to download them. For games that just have a PC release there only be a 'PC Download' button next to the game/episode but if there's a Mac version available there would be a button with 'Apple [the logo] Download' on it.
Currently reading... The Servants of Twilight by Dean Koontz
"Time is an illusion. Lunchtime doubly so."