Joe's right. It's not threaded correctly. It goes around the tension, over the little wire and then up to the take up lever (the thingy with the hole in it that goes up and down) and then to the needle.
The photo was too big to copy, but
here's the free manual for it. It has complete threading instructions as well as most everything you need to know about your 15-91. It makes me want to get mine out and sew with it, as that's the machine I learned to sew on.
bkay