iTunes App Store sells apps only for iPhone/iPad/iPod.
Unfortunately we have no way to transfer licenses since the Mac App Store and the iOS App Store are not connected.
Mac and iOS (iPhone/iPad) are two separate platforms and apps for one do not work on the other.
If you would like to purchase the Mac OS X version of iReal Pro, you need to go through the Mac App Store. Go to the Apple menu in the top left corner of your Mac screen and choose "App Store…"
If you purchased the wrong version of iReal Pro, you need to contact iTunes Support to request a refund: http://www.apple.com/support/itunes/