Thanks towo!
I almost got into trouble ...
First (with no d-m repo enabled) apt-get remove mencoder mplayer
If I did not do this, there was a dependency problem and apt-get -f install was going to rip out some dozens of packages.
After I did that, then I enabled the d-m repo, and avidemux came in with no problem.
Thanks again -- we have here a great helping forum!