From a terminal run these 2 commands:
echo "deb http://packages.medibuntu.org/ dapper free non-free" | sudo tee -a /etc/apt/sources.list wget -q http://packages.medibuntu.org/medibuntu-key.gpg -O- | sudo apt-key add - && sudo apt-get update
echo "deb http://packages.medibuntu.org/ edgy free non-free" | sudo tee -a /etc/apt/sources.list wget -q http://packages.medibuntu.org/medibuntu-key.gpg -O- | sudo apt-key add - && sudo apt-get update
echo "deb http://packages.medibuntu.org/ feisty free non-free" | sudo tee -a /etc/apt/sources.list wget -q http://packages.medibuntu.org/medibuntu-key.gpg -O- | sudo apt-key add - && sudo apt-get update