I tried using iTunes, but it only seems to have downloads for iPhone and iPad only. How can I get a version I can play on my iMac?
The App Store found in iTunes is only for iPhone/iPod/iPad apps. Mac apps can be found on the Mac App Store. Angry Birds is available for Mac for $5.
Note that iPhone and iPad apps are not linked to desktop Mac apps, so when you buy an app for an iPhone or iPad, it does not come with a Mac version and there is no guarantee that a Mac version is even available.