To unlock the Cell Games, you first must unlock it in the Dragon Arena. To do this, Cell must break in. You will need to fight a character level 51-55 for this random event to happen, and your fighter must be at a lower level than your competitor. Your character will need to be at least a level 25 to unlock fights against level 66 opponents. You will see an extra scene before the loading scene if he has broken in. If you don't get this, you can simply quit and return to the character select scene and try again until it happens. As this is random, it could take any number of attempts to happen, but will eventually. Beat Cell to unlock the tournament, which is found with the World Championships. It is a five round tournament, which shouldn't pose you too many problems.