Purchased the game for the wrong OS.

[jens]

The copies of FG's store retail, steam and app store are not interchangeable. You need to purchase it for your own platform/operating system.

In your case, you bought the Windows version. You need to purchase the OS X version. Keep in mind, that the Frictional Game's store offers all three versions of the game with one purchase (Windows, OS X, Linux). That's not the case with steam, amazon or app store of course.

You get all platforms with Steam.