Ok, Now I get it

At first the App did render the video to a small square on the desktop, instead of fullscreen. Then I had to ask the developer via facebook page, how to solve the issue - and restarted the app, as suggested: Then it worked. However, this should be present on the Apps documentation website, and a restart should be made automatically after choosing the quicktime video. Anyway, it did what I brought it to do. Just need some polishments.

Lady Jenny Stark about Mach Desktop, v2.7.2