You need to have it at night. And I believe you need a gba transported Pokemon for it to work. I'm not sure what Pokemon though.
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tap the tv at night. it will get rotom out.
You can catch a Rotom at night by talking to the tv
152: rotom (platinum pokedex) 152:chickorita (national pokedex) you can catch rotom when you examine the tv in old chateau between 8pm and 3:59am
You can get rotom by in the haunted house near eterna city and after 8pm and go to the one witch has the scary picture on and press A at the tv and you should get rotom. Rotom can be caught by a normal great or ultra ball
Pokemon #152 in Pokemon Platinum is Rotom. He can be found inside the Old Chateau which is located in the norhtern end of the Eterna forest. (Right near the Eterna City exit.) Go up to the top floor of the Old Chateau and then Rotom is in the second room from the far left. Interact with the TV and Rotom should start a battle.