After you beat the game (Beat the Elite 4), you can go to Route 15. It's across Marvelous Bridge. Enter the building up the stairs. This is the PokeTransfer Lab. To use it, you need 2 DS's. Talk to the guy at the top of the stairs, and he'll tell you how to function it. It's a minigame, you shoot pokeballs at the desired Pokemon that you're migrating. This way, you can receive pokemon from earlier games like Diamond, Pearl, Platinum, etc. You can get Arceus through the Poketransfer from earlier games.