1. Follow the official instruction to install all the restricted formats:

   sudo apt-get install ubuntu-restricted-extras

2. Install MPlayer:

   sudo apt-get install mplayer

3. Download MPlayer binary codecs package from here.
4. Install MPlayer binary codecs package: (Note: the package name may differ from yours.)

   tar xjvf essential-20071007.tar.bz2
   sudo mkdir /usr/lib/codecs
   sudo cp essential-20071007/* /usr/lib/codecs

At this point, you should be able to play rmvb files now. In case you still can’t, install libstdc++5 as follow:

sudo apt-get install libstdc++5