Where do you get Rotom in Pokemon Pearl?

The Pokemon Rotom can only be found at night, by entering the spooky mansion in Eterna Forest, going up the stairs, and then entering the middle door. Find the door that leads to the room with the TV, then walk up to it and press A. The game will tell you that there is a Pokemon hiding in the TV, and then it will ask you if you would like to thump it. Say yes and Rotom will appear.