You will enter the elevator shaft. Ride the elevator all the way down until it stops. Get off the elevator and wait for it to rise again. After it rises, jump down to the bottom and use first person mode to find the control box. Use a Batarang to disable it and the door behind it will open. Stay clear of the elevator and wait for the correct time to get in the open room. Inside the room is "Secret Key 2." Get out of the room and ride the elevator up to the top. At the top there is a door that is supposed to be locked. The key opens the door, and inside is an envelope with the letter "D" on it. Finish the game and the "Infinite Special Move Bar" cheat will be unlocked.