How do you use the batman skin in arkham city

After getting a costume, you can either complete the game's campaign which lets you select a batman skin or you can use a cheat code. You activate the cheat code by going onto the main menu (where you can select continue story, concept art, the credits, etc.) and press left left down down left left right up up down. After a confirmation sound, you should be able to select whatever skin you want before entering the game.