An idiots guide to finding this place, I hope.
If you look on your big map, you'll see what looks like a "wall" going through Hellfire Citadel, from north to south. The section of "wall" just south of Hellfire Citadetl and north of the road from HH is the entrance to BF. Just go up the stairs that are pressed up against that wall, follow it across, and you'll find it from there easily.
Hope that helps  =/