In Trisquel 6? You don't, I'm afraid (at least I don't).
Install Trisquel 7 instead (it's already usable, or at least the packages are since I don't install from the ISO), then try garsmith's suggestion:
sudo add-apt-repository ppa:gnome3-team/gnome3-staging sudo apt-get install gnome-session gnome-shell sudo apt-get update && sudo apt-get dist-upgrade
