Pokémon Global Link. Choose the event "Receive the Mythical Pokémon Arceus." It will take you to a password page where you enter in the code ARCEUSVOTE. Once entered, the site will lead you through how to send Arceus from the Global Link to your game.
This is not the first Arceus event, however, there was one for Platinum as well which was held at Toys 'R Us (in the US). The difference between these two Arceus are thier moves, OT, ID number, and Pokéball. The TRU Arceus will have the special moves Spacial Rend, Roar of Time, Shadow Force, and the level up move Judgement. The Dream World Arceus, however, does not have any special moves. It has the normal level up moves of Recover, Hyper Beam, Perish Song, and Judgement. Also, the Dream World Arceus will have the OT and ID of the person who caught it which means it can be Nicknamed and it can potentially be traded on the GTS. Both the TRU and DW Arceus are caught at Lv 100.
You know, it's intereseting that power pokémon like Arceus and Mewtwo are being released on the same month... Legendary throwdown anyone?