Now root user in Ubuntu is by default disabled. The reason for this is simple, if you login as root user and you did some horrendous or deleted some files in a hurry or by mistake your system might become unusable, but if you still would like to enable the root user and you know what you are doing then its straight forward. Open the terminal and add the following command
$ sudo passwd root $ <you will be prompted to add the root password twice>
add the root password. Now this will enable the root user in command line interface. If you wish to login as the root from the main system you will have to add the following line
sudo sh -c 'echo "greeter-show-manual-login=true" >> /etc/lightdm/lightdm.conf'