From Points East:  

If you're coming from Boston or Worchester or the surrounding area, you're best option is to take the Mass Pike west until I-91, then get on I-91.

If it's more convenient for you to get onto Rt. 2 instead of the Mass Pike, then Follow Rt.2 West into Greenfield MA.  Rt. 2 and I-91 become the same road for a couple miles, stay on I-91 South at this point.  Follow the directions listed below.

If you're in an area (such as Amherst, MA) where getting onto any of those roads would take you out of the way, then you're probably already somewhat familiar with the area.  Get on Rt. 9 and take it into Williamsburg MA.  Or if you're coming from directions South West, West, or North West, then you're looking at the wrong directions page, please refer to this: (Link)

Directions from I-91:

Note: The exit number is different for I-91 North and South, the first two steps are different:

I-91 North:
1: Get off exit 19
2: Go straight through the traffic light at the end of the off ramp, travel 1 mile and go straight through the second traffic light.
 

I-91 South:
1: Get off exit 20
2: Take an immediate right turn

Note: The directions are the same from this point
3: Follow this road until you come to a T-intersection with Rt. 9
4: Turn right onto Rt. 9 west.
5: Follow Rt.9 west for approximately 3 miles, Mad House Minis will be on your Right.

Alternate Directions:
Alternatively, you can get off I-91 (exit 19 from I-91 North, exit 20 from I-91 South) and just follow the signs for Rt. 9 West.  Following these signs will take you through downtown Northampton though, and they're not recommended. They are something you can fall back on if you need to.