Getting from the East Garage to 35-225:
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 right at the front of the foyer into Building 7 and straight through Buildings 9 and 33, all the way to the end until you reach Building 35.
As Bulding 33 changes to Building 35, you see a flight of stairs on your left and an opening to the right leading up a few stairs and into Building 35. Take the opening to the right, and Room 225 is at the end of the short corridor through the door at the top of the stairs.