The Hilton Mark Center Alexandria Hotel is located at 5000 Seminary Road, Alexandria VA 22311, just northwest of the Seminary Rd exit on I-395, five miles south of the DC line.
If you're driving, the hotel is located a couple hundred yards North of the Seminary Rd exit off I-395, just south of DC. Parking is $5 a day for attendees not staying at the hotel.
Check out the AR2013 Message Board which you can use to arrange carpooling and more.
If you travel by bus from Boston, New York, or Philadelphia, consider the discount bus services. They will drop you off at a Washington Metro station, where you can take a metro train to Pentagon City Metro station (blue/yellow lines) and catch the free shuttle to the hotel.
If you travel by train, from Union Station take the Metro Red Line to Gallery Place, transfer to Yellow Line to Pentagon City, then catch the free shuttle to the hotel.
Flying into Washington National Airport (DCA) is best. It gets you a free 10-min shuttle ride to the hotel.
From Dulles International Airport (IAD), a cab is fast and convenient for a group (35 minutes, $65). The cheap way for individuals is to catch a Washington Flyer bus to the West Falls Church Metro Station, take the Orange Line to Rosslyn, transfer to the Blue Line to Pentagon City, then catch the free shuttle to the hotel (1 hr, $12).
From Baltimore-Washington International Airport (BWI), a cab is fast and convenient for a group (1 hour, $80). The cheap way for individuals is to catch the B30 bus (pdf) outside Concourse A/B, or the International Terminal, to the Greenbelt Metro Station, take the Green Line to Gallery Place, transfer to the Yellow Line to Pentagon City, then catch the free shuttle to the hotel (2 hrs, $8).