I just installed Mint 16 and I see that root user is not available at login screen. I log in from normal user and went to "Login Window" option and there I set "Allow root login". Then I restarted the PC and still I don't see root user in login window.
I also did the below but it also didn't work.
sudo passwd root sudo sh -c 'echo "greeter-show-manual-login=true" >> /etc/lightdm/lightdm.conf'