Instructions
Beat the Elite Four at the end of Victory Road after you have defeated all eight Gym Leaders in "Pokemon Pearl."
Return to Sandgem Town and talk to the Professor. He will give you a National Pokedex as a reward. The National Pokedex allows you to encounter rare Pokemon that aren't normally found in the region.
Go to the Old Chateau at night. This creepy haunted house is located on the fringe of Eterna Forest. Be careful when walking around the mansion, as there are Ghastly, Haunter and Gengar Pokemon wandering the rooms at random.
Enter the middle upstairs room, walk up to the TV on the shelf and press the "A" button to provoke Rotom into a Pokemon battle. Rotom is only at level 15 at this point, so be careful not to knock it out by accident. This Plasma Pokemon is especially susceptible to Ghost and Dark-type attacks so use a low-level Ghost-type Pokemon to whittle down its health. Use a Pokeball to capture Rotom once its health falls to the red zone.