You mean the area where you put in cheatcodes? In order to do that you must get to the part in the game where are on Cid's airship with Yuna on your team. You should see Cid on the bridge so talk to him and you should see an option to put in your own passwords.
Type in Godhand to unlock the secret area where Riku's hidden weapon is.
Type in Victorious to unlock the secret area where Riku's special armor is.
Type in Murasame to unlock the special area where Auron's special weapon is.
Great, I'm moving. Who knows when you'll see me again.