1. Open the NetInfo Manager application in the /Applications/Utilities folder.
2. Go to the Domain menu, then drag down to the Security submenu, and choose "Authenticate...". Put in an administrator login and password and press return or the "OK" button.
3. Go again to the Domain menu, again to the Security submenu, and then drag down to the "Enable Root User..." menu item.
4. You probably won't have put in a root user password yet. It will prompt you to provide a root user password and verify it. Do so.
5. Root user is now enabled. Now just logout from the Apple menu, and then at the login screen, use the login name "root" and provide the password you just set.
This will log you in as the root user. Note that you can do ANYTHING when logged in as root, including deleting essential files. Proceed with caution.