I do ride the Norwottuck trail! In fact, I used it regularly to commute to Northampton. The problem is, I'm not really familiar with the trail out to Belchertown. It most assuredly does not end at South East St, though - I've been on the trail behind Amherst College, and that's just a short distance from South East St. I may have used the connector, but unfortunately I don't remember where it is. I would try striking north first. If you reach route 9/College St. without finding it, that was the wrong direction, and you should head south.
If you still can't find it, there's one other option: head north to route 9/College St. and turn left (west). Follow it across East Pleasant and down a long hill until you come to the traffic light at University drive. You should see the sidewalk on University drive, on your right, and across the street, on your left, has a yellow stripe down the middle. This is the UMass Bikeway Connector. Turn left and follow it for a couple hundred yards, and it will quickly spit you out onto the Norwottuck trail. From this point you can retrace the trail to find the crossing, or just keep heading west toward Hadley/Noho. I hope that made sense. I guess I'm taking the trail to Belchertown when I return to Amherst! I'd like to know where it goes.