Instructions
Beat "Dragon Universe" with each of the ten playable characters. You'll receive the Dragon Arena Ticket, which unlocks "Dragon Arena" mode.
Battle through the arena. After 100 fights, there's a slight chance that Piccolo will break in. The odds of him breaking in increase with each battle. If you're unlucky, you might have to fight over 300 battles.
Defeat Piccolo when he breaks in. This unlocks Cell Jr.