To unlock Mr. Satan, get everyone but Goku to Level 50 and then go through each character's Level 50 Area and collect their Character Trophies. Then beat the game and you'll get Goku's Trophy, with a message of Mr. Satan now being playable.

To get Gohan's Trophy, get to Level 50 with Gohan then go to the "50" gate outside Gingertown to collect the trophy.

To get Goku's Trophy, finish the game.

To get Piccolo's Trophy, get to Level 50 with Piccolo then go to the "50" gate at Namek planet to collect the trophy.

To get Trunks' Trophy, get to Level 50 with Trunks then go to the "50" gate at West City to collect the trophy.

To get Vegeta's Trophy, get to Level 50 with Vegeta then go to the "50" gate at Northern Mountains to collect the trophy.

You will want to get all the trophies BEFORE you finish the game. Then, BE SURE TO SAVE YOUR GAME RIGHT AFTER you defeat Cell. There is an alternate ending if you use Hercule! Saving your game right after you beat Cell allows you to see the normal ending, or switch characters to Mr. Satan and see the alternate ending.