First,install Mplayer by typing following code or going to Synaptic Package Manager:
sudo apt-get install mplayer
Download the codec package here,after mplayer has been installed.extract it by right click
In my case,I download the codec package and extract it to the Home Folder
So,open a terminal window, go to the file folder you extracted just ago and copy all of it to /usr/lib/codecs by following command:
cd essential..(press "tab" instead of "..") sudo mkdir /usr/lib/codecs sudo cp * /usr/lib/codecs
Go to Applications–>Sount&Video–>Mplayer Movie Player to open mplayer and right click at the mplayer-vedio window and select Preferences
Turn the Video tab and change the Available drivers: to x11 X11 (XImage/Shm).
At the Codecs & demuxer tab set Video codec family to RealVideo decoder and the Audio codec family to FFmpeg/libavcodec audio decoders.
Locate a rmvb file and right click to go to Preferences.
Turn to Open with tab and tick Mplayer Movie Player,then close it.