To unlock Hercule, you need to use Piccolo's Dragon Universe for at least the second time. BEFORE fighting Buu, but AFTER the "event" at Central City, search the desert near Baba's Palace (if memory serves...to the right/East). After you talk to Hercule, go back to Central City. After your conversation with Videl, ercule will become unlocked.
Death is but a door, Time is just a window,...I'll be Back!!!!!