One way to gain root prompt is to launch a shell as root. Therefore, using sudo you can type:
:~$ sudo sh
Password:
sh-3.1#
And there you are, you have a root shell.
If you want to be able to login as root, you need to define a password for root:
:~$ sudo passwd
Password:
Enter new UNIX password:
Retype new UNIX password:
Now you can login as root with su.
Re-disabling your root account
If for some reason you have enabled your root account and wish to disable it again, open a terminal and issue the following command:
sudo passwd -l root
