If you want to play rmvb in Ubuntu you have to do some easy steps, which i regularly forget about, so here is a description which will help me (and possibly you) to remember how it works.
One thing you should note is, that you will have to install non-free Components.

Only things needed should be a player like mplayer and the non-free package w32codecs.

Before you can install the last one you will have to add the medibuntu repository to your list:

sudo wget http://www.medibuntu.org/sources.list.d/gutsy.list -O /etc/apt/sources.list.d/medibuntu.list

now add the gpg key:

wget -q http://packages.medibuntu.org/medibuntu-key.gpg -O- | sudo apt-key add – && sudo apt-get update

Now install my favorite rmvb player:

sudo apt-get install mplayer

and finally the codecs:

sudo apt-get install w32codecs

To start playing from the console just write something like this:

gmplayer video.rmvb

if you dont understand anything above, just paste the given codes in the terminal and press enter.

main article: http://my.opera.com/dduenker/blog/2007/11/03/how-to-play-rmvb-in-ubuntu-7-10-gutsy-gibbon