BeckyMc - Yours is the 15-91 which has the "potted" motor (rather than the belt driven 15-90) The earlier models were treadle or handcrank.
Here's a site I have found useful in identifying Singer machines:
http://www.sandman-collectibles.com/...machines.htm#8
I was going to give you the ismacs.com site for a manual but see that miriam already has.
Let us know if you have any more questions.