I just installed unity on ubuntu 15.10 (willy werewolf) using the command :
sudo apt-get install ubuntu-desktop
it went good, but I can't "activate" it, I still have gnome. normally I should find a way to change in the login screen but I can't find anything
Try installing the LightDM display manager and switching to it, then whilst logging in through it, you should be able to swap between Gnome and Unity.
sudo apt-add-repository ppa:lightdm-team/ppa
sudo apt-get update
sudo apt-get install lightdm
Then run
sudo dpkg-reconfigure lightdm
and select lightdm from the prompt:
