Flying

Hartford, CT (Bradley International Airport) ~1 hr Drive

This is by far the closest airport, but despite the name it's domestic flights only.  If you're coming from another country, you'd have to take a connecting flight to Bradley.

Follow signs for I-91 North, take 91 North to exit 18 for Northampton Center.

Boston, MA (Logan International Airport) ~1.5 hr drive

Boston is the easiest airport if you're flying in from another country. You can get a direct flight from pretty much anywhere, and it's an easy drive to Montague.

Take I-90 West for about 90 miles, take the exit onto I-91 North continue for 12 miles and take exit 18 for US-5 toward Northampton Center. 

New York, NY (JFK) ~3 HR drive

Driving from New York is a bit more of a schlep, but it's not a bad drive by any means. Taking the Merritt Parkway is also a very pretty drive. 

Take I-678 N, follow signs for Whitestone Bridge, Continue onto Hutchinson River Parkway North stay on Hutchinson River Parkway and continue one CT-15 N, take exit 68 to merge onto I-91 N, continue for 50 miles and take exit 18 toward Northampton Center.

Newark, NJ (Newark) ~3.5 hr drive

Newark is often a little cheaper than JFK. It's much less convenient to get to NYC from Newark, but the drive to Massachusetts is comparable.

Take I-78 East follow signs for I-78E/I-9, take the exit for I-95 North, Cross the George Washington Bridge I-95 Lower Level, Take exit 1 on the left toward New York 9A North, take exit 4 for Cross County Parkway toward Hutchinson Parkway, stay on Hutchinson River Parkway and continue one CT-15 N, take exit 68 to merge onto I-91 N, continue for 50 miles and take exit 18 toward Northampton Center.