Instructions
Play "Pokemon Ruby," "Sapphire" or "Emerald" on the Game Boy Advance, and capture Registeel in the cave near Route 120. Save the game.
Start "Pokemon Diamond" on your Nintendo DS with the Game Boy Advance cartridge also inserted.
Select "Migrate From DS" on the main menu and then select Registeel from the list of Pokemon. It will only show up if you have already caught it in the Game Boy Advance game. Once you transfer it, you cannot return it to the other game.
Load your "Pokemon Diamond" game file and travel to the Pal Park on Route 221.
You will encounter Registeel randomly in the mountain area here; keep roaming around until you come across it.
Capture Registeel with the Park Ball you received when you entered the Pal Park.