Golden Balls
At the title screen, hold [I] + [Select] and press [Left] + [Up].
Passwords
Enter the password in the password menu at the title screen:
677261 - Area 5, Round 50
075653 - Fight enemy opponents only
573300 - Area 1, Round 1 (2nd Playthrough)
344710 - Area 1, Round 1 (3rd Playthrough)