If you haven't already, look at the manual which is accessible from the help menu in Zwei-Stein. It is at least the beginnings of a manual.
There is also context sensitive help. Point with the mouse to an area on the interface and press F1. The help will appear in the help window.
You can also try the "internet based" tutorials. They can be accessed when you start Zwei-Stein up, in the help area. Here is one that is transcribed to a webpage if you want to have a look. It also explains where to click for the "internet based" tutorials.
G.A. Deezen sent a link to his "Shadowrama" tutorial that he made for MST3K fans. It describes the use of a brightness keyer, key edge smoother, colour adjuster and how to adjust the size/position of the overlaid video.
If you need a tutorial in French, we can recommend one that was written by Jean CARTIER, a multimedia project manager and programmer in France.
Click here to download this 857 KB pdf file.
Searching the Forum for what you're looking for might be a good idea at this point if you are still stuck.