Ok, after you beat the Elite Four, go to the dungons where you weren't allowed in, near, Cerulien or something xD Well, anyway, you go through until you get to a dead end aggainst the water. (keep going through the cave, make sure you have a pokemon that knows surf.) Surf down along the channel to the right, go up on land then save your game, then talk to the pokemon standing there.
~Kat