Start at the East Garage, facing out from the ramps where you'd drive in. The building you're facing shows on the map below as a comb-shaped structure. Below this on the map, to its left as you face it, is a building which houses a swimming pool.
Walk between these two 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 to Building 3, just before the large foyer at the main entrance of MIT.
Enter the corridor to Building 3 on the left. Room 133 is the first room you pass on the right, up a tiny flight of stairs.