Why only for iOS 4.3
 

The euphoric me has bought Puzzle Agent 2 for his iPod 2G yesterday because it loved Puzzle Agent 1, just to find out it does not work (should have read the store description, eh?)

Why doesn't it run, what has changed, any way to get it to run or... any way to get my money back?

Refunds would have to be done through Apple, as it's their storefront you purchased it from. As for getting it to run, you could try performing an update through iTunes to get a newer version of iOS installed (Forgive my general lack of knowledge of Apple's devices), otherwise you're SOL.