If I remember correctly, Mephis 3.4 uses regular .deb packages.
From your terminal prompt, do $ cat /etc/apt/sources.list
If it looks pretty much like this:
Code:
deb http://ftp.us.debian.org/debian/ stable main contrib non-free
deb-src http://ftp.us.debian.org/debian/ stable main
deb http://security.debian.org/ stable/updates main contrib non-free
deb-src http://security.debian.org/ stable/updates main
...use an editor to add this line
Code:
deb http://www.debian-multimedia.org stable main
Then, as root, two commands; "apt-get update" "apt-get install avidemux" ... and you're done.
If you run into errors along the way ... ask!