Yes I read on another forum, that few people know where the entrance is and even thinking of the layout of Frontierland now I can't picture where it would be.
I think its beside Pocahontas Indian Village. We saw keelboats on the water when we were there, but it seemed to be only staff perhaps giving them the final checks before putting them back into service.
The entrance is situated between BTM & Pocahontas Indian village. However this is not anywhere to be found on the 'official' park map. This probably explains the very short queues..