avidemux (a free video editor designed for simple cutting, filtering and encoding tasks) is currently missing from Trisquel repos since it's in the multiverse Ubuntu repos (http://packages.ubuntu.com/natty/avidemux).
This is because avidemux is in the Ubuntu multiverse (i.e. non-free) repository. For whatever reason Ubuntu has classified this program as non-free so we will not be getting packages for it. The only way we would be getting this package is if its freedom status changes and it is moved to their free repository.
Note that the package avidemux-plugins-common from Ubuntu's repositories depends on the non-free libfaac0 and thus cannot be included in Trisquel.
Marking as "won't fix".
This is because avidemux is in the Ubuntu multiverse (i.e. non-free) repository. For whatever reason Ubuntu has classified this program as non-free so we will not be getting packages for it. The only way we would be getting this package is if its freedom status changes and it is moved to their free repository.
Avidemux is in Arch and Parabola repositories (Arch has faac as optional dependency) so I'm guessing it could also be packaged for Trisquel.