At the opening menu, press Triangle + Start + Circle + Select. If you do the code correctly, you'll hear "I'm finally a DigiMon master!".

Digivolve into Anything:

While your Digimon is Digivolving, hold Start + Select + Triangle + Square + X + Circle. When you see the Digimon's name flash, press Square. Then choose your Digivolution from the list.