From Points West:  
If you're coming from points west (Pittsfield area) then get onto Rt. 9 East until you get into downtown Williamsburg, MA.  Mad House Minis will be on your Left.

For points further West (Troy, NY, Albany) Your best option would probably be to get onto I-90 East and take that to I-91 North, then follow the directions here: (Link)

From Points South West:
The further away you are, the better the option is to take your most direct rout to I-91 North or the Mass Pike (I-90) East.  From there you should follow the directions here: (Link)
Other than that, the best route to take would probably be to try to get onto Rt. 66 and take that into Northampton.

From Points North West:
There's two main options: Rt. 2 and Rt. 9

Rt. 9 is simple because once you get on Rt. 9 East, you simply follow it until you get into Williamsburg.

Rt. 2 becomes a better option the more North you get, Probably a good option for those coming from North Adams.
1: You would start by taking Rt. 2 towards Buckland MA
2: In Buckland get onto Rt. 112 South.
3: Follow Rt. 112 South until the T-intersection with Rt. 9
4: Turn Left onto Rt. 9 and follow into Williamsburg, MA
5: Mad House Minis is on your Left.

There is also the option for following 112 instead of Rt. 2, This is ideal for those near Wilmington, Vermont.

For Points Further North or Further West:
Depending on how far North or West you're located, the best option rapidly becomes to get onto the nearest highway (I-91 or I-87).  For I-87 you would follow that until I-90, then follow I-90 until I-91.  Once you're on I-91, follow the directions here: (Link)