Getting from the East Garage to 66-148:
Building 66 is directly in back of the East Garage parking lot, but the closest doors are usually locked in the evening, so you'll likely have to enter through Building 16 and double back.
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 right and continue all the way to the end, where it enters Building 66.
Room 148 is near the end of the building, on the right.