To install VLC media player and MPlayer on Raspberry Pi and Ubuntu, issue these at the command line:
sudo apt-get install software-properties-common
sudo apt-get install python-software-properties
sudo add-apt-repository ppa:videolan/stable-daily
sudo apt-get update
sudo apt-get install mplayer vlc
The first two lines are for installing add-apt-repository. It may not be strictly necessary to install both (add-apt-repository is only in one of them, depending on particular versions of the operating system), but in this way it is guaranteed to work.