Hopkins Parking
Neil,
Driving north on route 73, just after the left turn into the main entrance of the Ausable Club in Saint Huberts and immediately before the bridge that crosses the East Branch of the Ausable (shortly after), there is a wide shoulder on the right for off-road parking. From there you can link with the Mossy Cascade Trail to Hopkins. If you cross the bridge, you've gone too far. Often (especially on weekends) the limited parking at that roadside location is quickly occupied.
There used to be a very old green wooden sign with carved letters near the start of the Mossy Cascade Trail. I think it said "Mossy Cascade" or "Hopkins".