Instructions
Defeat the Violet City Pokemon Gym Leader to get the Zephyr Badge. This makes the Pal Pad available, which lets you link with other gamers.
Catch a Magneton in the Safari Zone or the Cerulean Cave.
Go to the Pokemon Center in any town or village and go downstairs to the Wi-Fi Club. Talk to the assistant, Teala, and she will give you a Pal Pad. Log on to the Nintendo Wi-Fi Connection and you will be given a unique 12-digit number -- this is your Friend Code.
Start up "Pokemon Diamond" or "Pearl" on the second Nintendo DS and head to a Wi-Fi Club in a Pokemon Center. Log on to the Wi-Fi Network and get the Friend Code.
Enter the other game's Friend Code into the Pal Pad in each game. You will now be able to link the two games together. Do this and select "Trade." Trade the Magneton from "SoulSilver" across in exchange for any Pokemon.
Take the Magneton to Mt. Coronet, the mountain which runs right down the middle of the Sinnoh region between Eterna City and Celestic Town.
Go inside any of the caves and use the Magneton in the battles which occur with wild Pokemon until it accumulates enough Experience Points to level up and become Magnezone.
Go back to a Pokemon Center Wi-Fi Club and connect the two games again. Trade the Magnezone back to "Pokemon SoulSilver" and return the Pokemon swapped in the last trade to "Diamond" or "Pearl."