Some of our Mac users have found they can play it on Steam's Win version.
"If the game does not start from the Steam app because it says it can not find the file ‘/Users/.../Game.exe’, use the Terminal to go to the specified folder and do ‘open game.app’."
Steam will offer refund for in-game purchases within any Valve-developed games within forty-eight hours of purchase, so long as the in-game item has not been consumed, modified or transferred.