before trying to login as admin or root, fist you need to chose a password for it:
passwd root your pass repeat your pass
then copy and past the fallowing code
sudo sh -c 'echo "greeter-show-manual-login=true" >> /etc/lightdm/lightdm.conf'
then reboot your pc
next time that you want to login there will be an other box
for the username chose root
and for the pass, the one that you set before