Things You'll Need
Instructions
Method 1
Put the event Regigigas in your party in Platinum. This special Regigigas could only be obtained in 2009 by attending a Toys'R'Us special giveaway event. If you did not attend the event, you must trade for a real event Regigigas with someone who did.
Go to Iron Island and enter the cave. Travel through the cave until you reach the final rooms, and a new cave, the Island Ruins, will unlock with Registeel inside. Capture it during your fight.
Go to any Pokemon Center in both Pearl and Platinum using the two Nintendo DS systems. You can then trade Registeel to Pearl via the wireless trading system.
Put both Pearl and Emerald, Ruby or Sapphire into your Nintendo DS, and begin play of the Game Boy Advanced version.
Head to Route 120 with a Pokemon that knows how to surf in your party. A cave known as the Ancient Tomb is located in the northern section of this route. Enter it and capture Registeel during your battle.
Save and quit your Ruby, Sapphire or Emerald version and begin to play Pearl. Open the pause menu and there will be an extra option for transferring Pokemon. Select it, and it will prompt you to choose six Pokemon to transfer from the Game Boy Advanced version. Registeel should be one of them.
Head to the Pal Park, which you gained access to from Professor Oak upon defeating the Elite Four and obtaining the National Dex. You will be given Pokeballs that allow you to recapture the six Pokemon in the park without having to fight them.