Instructions
Find a person who has a Arceus and is willing to trade it. This can be done through the Internet at sites like friendcodes.com, or anyone in your area who plays "Pokemon Diamond," "Pearl" or "Platinum."
Locate your friend code in your "Pokemon Platinum" game. It is located in the Pal Pad inside your inventory.
Exchange friend codes with the other person and decide on a time to meet in-game to exchange Pokemon.
Wait for the arranged time, start your "Pokemon Platinum" game and head to the downstairs of any Pokemon Center.
Speak to the lady standing in the center behind the counter. Afterward you will be taken to a room where you can view a list of your friends that are online.
Select the friend who is willing to trade Arceus and press "Trade" in the menu. Find the Pokemon you agreed to trade in exchange for the Arceus and the trade will begin. Once the trade is complete, you will have your own Arceus.