Take the Red Line to the Davis Square T stop. From there you have to go north on College Avenue about 1 mile. You can walk or take a 96 bus to Medford Square. If you take the bus, get off at the intersection of College Avenue and Boston Avenue. Continue north on College Avenue and cross the railway line. Halligan Hall is the large brick building on your left (it's a former radio factory).
The entrance for the CS department is at the southeast corner of the building; that's the corner closest to you. The pictures predate our 2013 renovation; some colors have changed.
My office phone number is 617-627-4923.