To install mencoder in Debian, e.g. for etch:
1. Add "deb http://www.debian-multimedia.org etch main" to /etc/apt/sources.list
2. gpg --keyserver subkeys.pgp.net --recv-key 07DC563D1F41B907
3. gpg -a --export 07DC563D1F41B907 | sudo apt-key add -
4. apt-get update
5. apt-get install mencoder