Highlight "Exit" selection on the options screen. Then, hold X and press B, A, RS. Release X, then press X again. Fidex will bark to confirm correct code entry. Alternatively, hold X and press P, A, RS, A, LS(2), A to activate the code. Zack will say "Digital" to confirm correct code entry.
At the options menu, highlight Exit and hold X, then enter the code: