To Get Rotom You Have to have goten through the Pokemon Leauge, and obtained The National Pokedex. After That Go to the Eterna Forrest and you see Gardenia (the 2nd Gym leader) Infront of the Old Chateau. Talk to Her and then go into the Mansion. Go up the Stairs and go into the the very back door. Then Go one door to the Left and you should see a TV. Go up to it and and hit the A button, Then it should say: INSIDE THE TV... THERE APPEARS TO BE A POKEMON? THE POKEMON APPEARS AS IF IT WANTS TO COME OUT, SHOULD YOU THUMP THE TV? And then answer Yes and Get ready to battle Rotom. If this Dosn't Happen Check back in an hour or so, it might not be late enough.
AND REMEMBER ROTOM WILL ONLY APPEAR AT NIGHT.

ENJOY!