From the south:
Take I-90 to I-95 (exit 15, I believe).  (You can also take
route 20 out to 95, if you know that route, otherwise I'd
just suggest taking 90.)  Take I-95 north, to I believe 
exit 27.  (Well I'm pretty sure it's 27 from the north, at least)
Before the exit you will pass Microsoft, Parametric Technologies,
Oracle, and the Westin Hotel will be about at that exit.
The exit sign may be labelled winter St. and probably says
Waltham, but I'm not sure.  Anyhow, after the exit, take
a right.  4th ave. should be a street or two on the right.

From the north: Take route 2 to I-95 south.  Take the exit for Winter
St. (exit 27, I believe).  It will only be 2 or possibly 3 exits.  The
exit will take you in the wrong direction, so you'll need to turn
around.  I believe by getting in the far left lane you can make the
U-turn.  Cross over I-95 and 4th ave. should be a few streets on
your right.