Miriam,
The bobbin is similar to the one on my Wilcox and Gibbs industrial lock stitch machine. It has a weird convoluted method of threading it. Is it possible you've missed a step or a hole in threading this one?
My wife says the cams should just slide over the knurled knob over the other cams. I think the knurled knob should unscrew to switch the cams. We're just guessing.
Joe