My office is E14-574K. E14 is the new Media Lab building, my office is on the fifth floor and is #574K, on the east side of the building (MIT Medical Center side.)
The Media Lab is located at 75 Amherst St., Cambridge. This is close to Kendall Square, off the Red Line. From the Kendall Sq. T stop, you can head up Main St. away from the river, take a left at Ames St., and go to the corner of Ames St. and Amherst St. The gray building is E14.
Here's a handy map!