Start at the East Garage, facing out from the ramps where you'd drive in. Follow the path between the two facing buildings, then diagonally along the walkway to Building 26 on the left. Pass through Building 26 and several sets of doors to the end, where you'll find a staircase.
Go up the stairs and through the short hall, and turn right at the Infinite Corridor. Follow the Infinite Corridor a short distance to Building 4, on the left.
When you find Building 4, go up the stairs to the third floor and follow the corridor to the right. Room 370 is all the way at the end.