How to Enable the "root" Account on Mac OS X

For Mac OS X 10.5 and later you can use the Directory Utility instead of NetInfo. For just Mac OS X 10.5 you will find it in the Application/Utilities folder. For Mac OS X 10.6 and later you start the Directory Utility from the System Preferences as follows:

1. Start System Preferences and select "Accounts".
2. Under the list of users press the "Login Options", and then next to that press the "Join..." button next to Network Account Server. (You will not be joining a network account server, this is just the convoluted way to get to the Directory Utility).
3. Press the button to "Open Directory Utility..."

Once you launch the Directory Utility, however you do it, then enable the root account by doing the following:

1. Click on the Lock icon and give the Administrator password to authenticate.
2. Pull down the "Edit" menu to "Enable Root User"
3. Pull down the "Edit" menu to "Change Root Password..." and enter a good, strong password for the root user.

Reference :
http://www.spy-hill.com/~myers/help/apple/EnableRoot.html