Here is a link (I hope) to a thread that was started on this board quite a while ago but if you can get it, it might help.
Morse Fotomatic 4100 Zig-Zag Sewing Machine Manual
Welcome to the Board! It's great that you want to use this machine and want to find out how to do that so you don't mess it up. Good luck!
edited: I don't think that link worked so try this:
http://www.quiltingboard.com/vintage...l-t196349.html