Apple has, yet again, changed the method for enabling the root user in Snow Leopard. This time, the Directory Utility in Leopard's Utilities folder has been moved (buried) in the Accounts section of System Preferences.
In Leopard, you start at the Directory Utility in /Applications/Utilities.
After launching the app as an admin user, you go to the Edit Menu and there's a Menu item entitled: "Enable root user." Easy enough.
Directory Utility, Edit menu
That must have been too easy in Leopard, so Apple buried it in Snow Leopard ever further. You must be using an admin account:
1. Go to System preferences.
2. Select Accounts.
System Preferences -> Accounts
3. Click on Login Options.
4. Under Network Account Server, click on "Join."
Yet more techie details
5. When the new window slides down, click on "Open Directory Utility."
6. Now you can select, from the Edit Menu, "Enable Root User."
Back where we belong
Sidebar: Apple has, I surmise, changed the method to bury this function even further. Each step results in an ever more alarming and technically imposing dialog box designed to keep all but the expert (or most steadfast tinkerer) away. The reason, of course, is that enabling the root user is a less secure way to operate the Mac. Some users elect to run as an admin user and some as an unprivileged standard user, but one should never login as the root user for routine work because of the damage one could inadvertently cause to the OS.
This facility is there for experienced UNIX users who know what they're doing. They know how to do what they need to do, then get out, then disable the root user.
Normally, the installer of Mac OS X is the admin user who has plenty of privileges to get things done, but doesn't have complete root privileges so that the system integrity and security is maintained. That admin user is in the sudoers list (/etc/sudoers), so if necessary, some UNIX commands can be conducted on the command line with "sudo."
I've added this sidebar so Mac users learning UNIX or newbie IT managers new to the Mac can get a feeling for what's going on. For those who aren't into UNIX and don't use the command line, you'll just want to stay away from this feature altogether.