Enter the first password at the password screen, then press Start. Then go back to the password screen again and enter the second one, and press Start. A sound effect will confirm success and Debug will be added as the third option on the title screen.
The passwords differ from European to South American version.Here follows the codes for the latter version:
If you have no yin yangs at all and find two or more of these, don't get them all at once. Get only one them and transform. You'll see the yin yang you just got renew itself. Now, get them all and you'll be off with more yin yangs than you would have if you got them all in the first place.