For $8, you can park in the Cousens lot right across from Halligan. (If $8 is too much, there are parking kiosks for street parking in Medford, and there are meters and even a few free spaces in Somerville around Davis Square; you can park for two hours for $2. The T directions will tell you how to walk from Davis.)
In case of snow, you can also park in the Dowling parking garage, which is on Boston Avenue just north of College Avenue. 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.