Are you at the main menu or actually in game when you hit F1? You should have actually loaded up a world and be in it when you do. Also, are you hearing "Trainer activated" twice? When you hit F1 and hear it the first time it is scanning the game code. It will say "Trainer activated" again when it is ready to go.
Just point out here as well as in the trainer post. This fact apparently means that the trainer page needs to have system requirements updated, seeing as how no one using XP can use it. .Net 4.5 won't install on XP, only on Vista and later.