Ouvrez un terminal et tapez :

 wget -q -O - http://archive.getdeb.net/getdeb-archive.key | sudo apt-key add -

 sudo sh -c 'echo "deb http://archive.getdeb.net/ubuntu trusty-getdeb apps" >> /etc/apt/sources.list.d/getdeb.list'

sudo apt-get update

sudo apt-get install freemind