Getting from the East Garage to 1-390:
Standing in the East Garage parking lot, with your back to the entrance on Main Street, walk all the way to the back of the lot and around Building 66 to the left. Turn right at the back of Building 66 and follow the path all the way to the entrance to Building 56; the various doors before that are most likely locked and unuseable without an MIT student keycard. Enter the door into Building 56.
Follow the corridor to the left, and continue all the way to the end.
Turn left, then go up the stairs and through the short hall, and you come to the Infinite Corridor.
Make a right turn into the Infinite Corridor and follow it to the end, where it 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 leads directly to the Bechtel Lecture Hall, room 1-390.