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 the end, which comes out into a large foyer. This is the main entrance of MIT.
Follow the corridor on the left at the front of the foyer into Building 5. Follow this all the way to the end; this will lead into Building 1. Continue around the bend to the left and all the way to the end.
Take the stairs to the third floor. This should lead directly to the Bechtel Lecture Hall, room 1-390.