If you are wanting to purchase Keynote for your computer, then you are not selecting the correct Keynote application. The one you are selecting is for an iOS device (an iPhone / iPad), and that is why is it telling you, it needs iOS 5.1.
Keynote for the computer, is purchased via the "App's Store". Click on your "apple" in the top left, and from the drop down, select "Apps store". When the window opens, type in the words "Keynote" into the top right corner. This will bring up the version of Keynote you require fro a computer.
The current version of Keynote requires you to have 10.7 OS. Your notes state you have 10.6.8. If this is correct, then you will need to update your OS, in order to get the latest version of Keynote.