Saibamen: In the Dragon Arena: use a Level 10-20 character, and battle opponents *AT LEAST DOUBLE* your own level. Eventually, Nappa will "Suddenly Break In". Defeat him, and the Saibamen will become unlocked. *Example: A level 25 Cell should battle a Level 50 anyoune. Once you "Power-Up" to Level 26, battle a Level 52,...and so on.
Cell Jr.: In the Dragon Arena: use a Level 20-40 character, and battle opponents *AT LEAST DOUBLE* your own level. Eventually (most likely around the time you reach a Level 36-40,...provided you follow the example listed above for the Saibamen), Piccolo will "Suddenly Break In". Defeat him, and Cell Jr. will become unlocked.
I just copied this from my walkthrough,...in case anybody was wondering.
Death is but a door, Time is just a window,...I'll be Back!!!!!