How do you Evolve Rotom in Poke'mon Platinum?

Answer 1

Rotom doesn't evolve, it changes forms. You can make it change forms by using the secret key in the Galactic Eterna building's first floor in the upper left-hand corner.

Note: you must use it on the strange machine in the corner, and not on Rotom.

Answer 2

You need the Secret Key. You can get it over the Wifi as a gift, or there is a code for action replay, too. Then when you have it go to the team galactic hideout with the three trees at the entrance, right outside of the forest. I think its Eterna city???? Then you go to the back left wall and hit the action button. A doorway will appear and inside will be the objects to allow Rotom to evolve.