Rotom is a lv 15 legendary after you beat the elite 4 so don't use your normal pokemon. After the elite 4 go to Eterna city and go out left. Keep going left until you get to the tree that goes to the forest. Cut the tree near the fence and go into the old chateua. Go upstairs and through the door in front then go through the second door and click on the TV. It will only work at night though.