I work in Lowell, so I figure I can give you pretty decent directions:
 
From Southern Mass., take Rt. 495N. Take the Lowell Connector, and go
all the way to the end, about 2-3 miles (there will be lights at the
end).
Take a Left onto Gorham St. (There will be signs posted along the way to
let you know the direction of the Tsongas).
Follow Gorham St. all the way in, going through 4 (or so) sets of
lights, until you come to be perpendicular to Merrimack St. There is no
other choice but to take a Left here, so take the Left.
Go through one or two more sets of lights until you come to a large
5-way intersection. (Merrimack St. will cross with Dutton St. here).
Take the first Right onto Arcand Drive, just after Cobblestones
restaurant at the right corner of the intersection.
On Arcand Drive, you will pass the Lowell Police Dept. on the Left and
Lowell High School on the Right.
Come to the lights at the intersection at the end of Arcand Dr. and you
will see the Tsongas straight ahead. Go straight through the lights and
park to the Right.