If you have Diamond, Pearl, or Platinum, you have to have gotten the event on one of those games. Then you would migrate. Also, with looking it up on the GTS, you have to have seen or owned one for it to be in your Pokedex. If you haven't owned one or seen one in an in-game battle (not wifi), then it won't be in your Pokedex, and therefore you can't search it on the GTS. If you didn't get the event though, then try Shocking Pichu's way.