By the way, because Rotom is nocturnal, you must examine the television at 8:00 P.M. or later (I'm not sure when night changes to morning, but you should obviously check the television before that time); also, as Turbonegro said, you must possess the National Dex first. Note that Rotom is not very powerful when you battle it, despite the impressive battle theme; since it is only Lv. 15, be sure to bring Pokémon capable of weakening it without knocking it out. If you're worried about your team being too powerful for it, try bringing one Pokémon with Foresight or Odorsleuth and another with False Swipe; if you use Foresight or Odorsleuth on Rotom first, then you can hit it with False Swipe to safely reduce its HP without fear of knocking it out. Because Rotom is so low-leveled and has a decent catch rate, however, you shouldn't have too much trouble catching it.