Yep, it was indeed confusing.. Took me a couple tries to find where the trail continues.

At the junction of the canal and river, after you cross the Erie Canal Path's truss bridge, cross Main Street and go along Niagara Street for one block. The entrance of the trail is on the right side, next to Seymour Street (and the path ducks under the Seymour Street bridge over the canal).

That path will take you all the way through Niawanda Park and Isle View County Park and beyond on the Niagara River. I don't like the path past Isle View County Park because after it goes under the Grand Island bridge, the path goes by a Niagara Mohawk coal power plant, and the sooty smell can get very strong to the point of causing nausea. If you continue on it will eventually take you to Lasalle Park on the shores of Lake Erie.

My usual ride when I lived in Amherst was to just go down the Amherst Tonawanda trail to Isle View Park and then turn back.

Hope this helps.