Castling is a king move.. so you have to move the king first to g1 or c1 and the rook will follow. If you have moved the king already you can't move it back to original square and then castle.
So move the bishop and knight(and queen if you castle queenside) out of the way and you are ready to castle. Good luck!
I don't have any experience with the chess app.
Does anybody know the approximate rating/strength of the Chess * (it's listed as Chess Free upon download) app in the Apple app store? Another completely unrelated question I have is, how do you castle while playing live chess here? I understand the concept, but I can't actually do it on the computer. Thanks for any help.