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