I agree that you need a zz cam on the stack to make it zig zag. I have a dressmaker that needs cams so it can do all it's fancy stitches including the zig zag. Which is annoying because I don't have any of the cams. LOL So really it's just a straight stitch machine until I can track down a set.
From what I can tell with the dressmaker cams there's different versions so your cams might not actually go with your machine. If the cams do go with your machine, then I would try widening the stitches. That is what I usually forget to do and then I start cursing the machine for not working right when it's really just user *coughmecough* error. lol
As for that plastic tab, check the underside of the top. It might be there as a spacer for something underneath it.