frudemoo,
There are two basic and a couple of sub classes of Singer cams. The flat cam and the top hat cam.
Here is a pic of each on my 401:
Some Singers require you to raise the needle out of the work, and move the stitch width lever all the way to the widest setting as you install and remove the cam. My 338 is like that and I think yours is too.
If you machine does not have any built in stitches it will need the #1 cam (if it uses flat cams) or the #O cam if it uses top hat cams in the machine to do ZZ stitches.
If there is no cam in the machine it's very easy to destroy needles. The cam keeps the needle bar from moving too far to either side.
The cams can be had from eBay and other on line sources.
Oh,and here's a site with lots of pics and descriptions: {
http://www.singersewinginfo.co.uk/pattern_cam_list/ }
Joe