Play The Arena Stage Without Any Cages:

This is a glitch more than a Code, but you can use it to play in the Arena stage without any cages.

1) Finish Time Attack Mode.
2) Play Story Mode using the SAME CHARACTER. When you reach the final stage, skip Heihachi's intro (ie. press Start).

When the round begins, you'll see that the cages are down (actually, they're invisible). You can leave the Arena by going to the ramp which Heihachi walked down in the intro.

Note: In order for this code to work, you must play as a character who actually fights in the Arena Stage during story mode (for instance, Jin never fights in the Arena, so the glictch doesn't work for him).