Getting from the East Garage to 16-154:
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.
Room 154 is on the first floor.