Things You'll Need
Instructions
Complete the game's story line and purify all 48 Shadow Pokemon. Make sure that you only have five Pokemon in your team, then save your game.
Turn off your Gamecube and remove the "Pokemon Colosseum" disc.
Insert the promo disc and visit the Colosseum screen. Choose the option on this screen for visiting the Sacred Shrine.
Watch the short scene that occurs and Celebi will join you as long as there is at least one empty spot in your team. Let the game save.
Turn the Gamecube console off, and remove the bonus disc. Put the "Pokemon Colosseum" disc back in and turn the console on. Load your saved game. Celebi should now be in your party.