Here are directions from what looks to be a semi-official site: http://www.harvardsquare.com/map.html#how
Be forewarned: parking in the area is a bear. Your best bet is just to pay for off street parking.
I just realized the directions I linked to don’t tell you how to get there from New Hampshire. I would take 93 S and get off wherever you see signs for Storrow Drive, (with all the Central Artery construction I don’t know exactly how you do this anymore) which you would take West until you get to the Harvard Square exit, and follow the directions from there.
If you want to try Mapquest the Harvard Coop which is more or less in the middle of the Square is at 1400 Mass. Ave., Cambridge.
Telemark, thanks for the link. And also thanks to the funny bunny for the directions to Harvard Square, that’s enough to let me find it.