I'm not sure if there is a way to get there from Rt. 28. We always go from the Merrymeeting side.
Starting at the Rt. 11 Alton Circle follow signs for Merrymeeting Lake (the road just to the right of McDonald's). When you get to the NH Fish Hatchery and the dam, take a left onto Merrymeeting Road. You will go past the Merrymeeting Market and Marina. Follow road for approximately 2 miles (guesstimate). The road takes a sharp right turn and becomes North Shore Road at that point.
Here at the start of N. Shore Rd, a dirt road branches off to the left. This is the road that leads to Devil's Den. You can drive in part of the way, but at some point the road degrades and you will need to park and go the rest of the way on foot.
http://www.mapquest.com/maps/map.adp...3d&ambiguity=1
I haven't been up there for several years so perhaps someone else can take over from here. But if you get this far, there is usually someone nearby whom you can ask to point you to the start of the trail.