Things You'll Need
Instructions
Start a new game in "Pokemon Diamond" or "Pearl" by selecting "New Game" from the main menu.
Go to Lake Verity as prompted at the start of the game, and select Piplup as your starter Pokemon.
Go to Sandgem Town in the southeast of Sinnoh, and speak to Professor Rowan in his lab to get a Pokedex.
Go to any Pokemon Center and go upstairs to the WiFi Club. Speak to the attendant to get a Pal Pad.
Load up "Pokemon SoulSilver" on the second Nintendo DS and head to any Pokemon Center. Go to the computer on the right of the main desk, and arrange your traveling party so it has a maximum of five Pokemon. Then head upstairs and get a Pal Pad if you don't have one already.
Open the Pal Pads in each game to get the Friend Code for each game. Enter the "SoulSilver" Friend Code to the "Diamond" or "Pearl" Pal Pad and vice versa.
Speak to the middle attendant next to the door in both games to gain access to the Union Room. Head inside and the trainer from each game will appear on the same screen.
Select "Trade" and transfer Piplup across from "Diamond" or "Pearl" to "SoulSilver." Piplup will appear in your traveling party.