To unlock the root account in a sudo based system, log in as a normal user, open a terminal and type
bash$ sudo passwd root
enter your sudo password and then follow the prompts to set a root password.
After this you can log in as root in a terminal although you may not be able to log in graphically as root as some login managers prevent it.
If you wish to occasionally use the root account but do not want to unlock it you can always type
bash$ sudo su -
whenever you need to log in as root. This will let you use your sudo password to log into the root account.
If you have unlocked the root account and wish to lock it again simply type
bash$ sudo passwd -l root