Instructions
Play through the main storyline of the game until you have earned the first gym badge. You must defeat the first gym master in Kurogane town to unlock Wi-Fi trading.
Have a friend capture a Rotom on her copy of "Pokémon: Diamond/Pearl." You can find Rotom in the television in the Old Chateau, but only at night.
Head to the second floor of any Pokécenter and speak with the middle woman to enter the Union Room. Your friend will need to do the same on her DS.
Speak with your friend in the Union room to initiate a trade. She can choose one of your Pokémon, and you can select her Rotom.
Confirm the trade to add Rotom to your "HeartGold" stable.