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).
This is my exact message to Steam support can anyone help me?
Umm I used your service and you guys scammed me $10. I downloaded farcry 2 and when i go to my library on your application it says this game is not available on your current platform. Well maybe tell me that before I purchase. Im on an iMac 2008 OSX.
If you can't help me I want my money back because this is ridiculous.
Model Name: iMac
Model Identifier: iMac7,1
Processor Name: Intel Core 2 Duo
Processor Speed: 2.8 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache: 4 MB