Take the Orange Line to the Virginia Square-GMU station. Take the escalator to the street level, and turn to face Fairfax Drive. Across the street and to the right, you will see the FDIC building. The Arlington Campus is next door.
Arlington County and the Car-Free Diet recently created a video that showcases the Arlington Campus and the ways to get here without a car. You can see the video here:
Take the Orange Line to the Vienna Fairfax-GMU Metro Station. After coming up the escalator and passing through the turnstile, take the left exit that points towards the bus pickup and take any of the CUE Buses ($1.75) to Mason's Fairfax Campus.