For $8, you can park in the Dowling parking garage, which is on Boston Avenue just north of College Avenue. (If $8 is too much, there are meters and even a few free spaces around Davis Square; you can park for two hours for $2. The T directions will tell you how to walk from Davis.) From Dowling, walk a half block southeast to College Avenue and turn left. Continue 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.
The entrance to Halligan Hall is near N 42°24.475' W 71°06.950'.
The garage at Dowling is at N 42°24.541' W 71°07.096'.
My office phone number is 617-627-4923.