You need to add the Marillat unofficial Debian multimedia package repositories to your /etc/apt/sources.list. I got the following instructions from
ubuntuguide.org - adding extra package repositories.
1. Open a root terminal (menu Applications -> System Tools) and do: gedit /etc/apt/sources.list
2. Scroll to the end of the file and paste the following:
deb
ftp://ftp.nerim.net/debian-marillat stable main
deb
ftp://ftp.nerim.net/debian-marillat unstable main
deb
ftp://ftp.nerim.net/debian-marillat testing main
Then save the file and exit gedit.
3. While still in the root terminal, do the following commands:
# gpg --keyserver wwwkeys.eu.pgp.net --recv-keys 1F41B907
# gpg --armor --export 1F41B907 | sudo apt-key add -
# apt-get update
# apt-get install libdvdcss2
You may want to run apt-get with the -s flag to simulate the installation first:
ie. apt-get -s install libdvdcss2