It doesn't look too positive about a refund. More than likely they'll tell you that the system requirements for the game, including the the required OS is at the bottom of the game's page.

I don't have Steam for Mac, but I take it that it's a lot like Steam for Windows in its layout. However it's questionable why Steam for Mac would even list games that are incompatible. You might be able to use that to persuade them to give you a refund (i.e.: if it won't run on a Mac, why list it in the Steam store for Mac).

Good luck