You need Packman (as you can actually see in the screenshot), the standard multimedia repo for openSUSE.
Add that in YaST->Software Repositories by clicking on Add and choosing “Community Repositories”, or use one of the “openSUSE multimedia” 1-click installs that are available on the net.
But what confuses me a bit: with only the four standard repos, you should not see any avidemux, not even the cli version…
Are you sure you don’t have an additional repo you didn’t tell about?