Take exit 31 to rt 4/225 west-bound. Stay on 225 for 3-4 miles. The ramp is on the right, just before you cross the river. It looks like a small dirt turn-out/parking lot from the street and there's a road leading off into the woods. If you hit the river, turn around and come back over as you missed the ramp =)
Just a note to those new to the area, if you approach from 95/128 on 4/225 through Bedford (where I work), you will have to make a left turn at the Volkswagen dealership to stay on 225 where it splits from Rt 4, very shortly after bearing right from the split from Rt 62. It doesn't look like a left turn on the map, but it is indeed a left turn and is in front of a gas station in the middle, right at the VW dealership. I hope that helps...