How do you get to the ghoul underground in fallout 3?

If you're referring to the quest given at Tenpenny Tower, the Ghoul underground is located in Warrenton metro station, to the southwest of the Tower. Roy Phillips is located there, along with some other ghouls.

If you're having trouble finding the station: the first time you discovered Tenpenny Tower, you'll remember that Roy Phillips was yelling into the intercom, and left after a short while. Walk in the general direction he was going, and you'll come to the metro station.