The most recognizable feature on MIT's campus is the "Infinite Corridor", the head of which is located at 77 Massachusetts Avenue, Cambridge, MA. GA will be in building 6-120, which is a short walk straight down the infinite corridor -- see the map below.

The best way to get directions is probably through Mapquest or Yahoo! Maps, rather than us giving you poorly worded and generic directions. If something goes wrong, you can blame the internet, not MIT.