Rotom is in the TV in the Old Chateau (Eterna Forest). It's the room on the ground floor, at the far left. Rotom only appears at night.
In Diamond/Pearl you also need to have the National Pokedex (after beating Elite Four).
In B/W and B2/W2 on Route 15, someone will be there asking for a Ditto in exchange for their Level 60 Rotom. The level of Ditto does not matter and will not affect the level of Rotom, unlike previous generations.