Strong Warning: You should only enable root if you really really need it, really know what it's for, and realize the security implications of enabling root! You can do everything you need to do with "su do", so root is really not needed ... with that said, here's how to enable it - please understand what you're doing and why before you do this!
The "root" user (also known as the superuser) is the most powerful UNIX account. The root account can do anything to any file or folder, anywhere on the system. For that reason, it's considered quite dangerous, and only needs to be used (occasionally) by advanced users.
Because of the dangers of operating as root, Apple has chosen to hide the root account in OS X Final. However, there are a number of ways to enable it. The easiest is to boot off the install CD, and look under the Install menu for the "Password Reset" option. You can use this to change your own password if you forget it, and to enable the root account. This utility will not run if you copy it to your hard drive! It only works when booting from the CD.
Read the rest if you'd like to know how to do this from within OS X, and skip the CD-based reboot.
The second easiest way to enable root is to start a terminal session, and type:
sudo passwd rootYou will be prompted for a new password to enable root access. Many find this easier than rebooting, but since it involves the command line, newcomers may find it a bit more intimidating.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20010324095804436