Sometimes you can find demo's of vintage machines on Youtube. Do a search there. I came up with this one that looks similar to your machine:
http://www.youtube.com/watch?v=oEYm5Z9UCZg
Looks like the knob on the base (teardrop shaped knob) is what controls dropping the feed dogs.
The zigzag could simply be the straight-stitch setting with the stitch width set to >0. (Since it has Morse Zig Zag on the logo, it's bound to be able to do a plain zigzag!)