Instructions
Insert a copy of "Pokemon Diamond," "Pearl" or "Platinum" into the secondary slot on your Nintendo DS. Insert "Pokemon SoulSilver" into the main slot and turn the system on.
Select "Transfer From" from the "SoulSilver" main menu. Select "Rotom" in the list of available Pokemon in your older game to make the transfer.
Load your "SoulSilver" game and travel to the Pal Park in Fuchsia City. Locate your Rotom in the park. When you encounter it, throw a Park Ball to capture it and add it to your collection.
Visit a Pokemon Center to add your new Rotom to your active party of Pokemon. Return to Silph Co.
Approach the elevator with Rotom in your party and the doors will open. You'll get a message indicating that Rotom's electricity is effecting the elevator and you'll be taken up to a secret room.