Enabling the root user in Mac OS X
Mac OS X v10.6 and later
1.From the Apple menu choose System Preferences….
2.From the View menu choose Accounts.
3.Click on the lock and authenticate with an administrator account.
4.Click Login Options….
5.Click the “Edit…” or “Join…” button at the bottom right.
6.Click the “Open Directory Utility…” button.
7.Click the lock in the Directory Utility window.
8.Enter an administrator account name and password, then click OK.
9.Choose Enable Root User from the Edit menu.
10.Enter the root password you wish to use in both the Password and Verify fields, then click OK.
Note: If you are troubleshooting an issue that prevents you from logging in as an administrator, follow the steps in this article to enable the root user.
Mac OS X v10.5
1.From the Finder’s Go menu, choose Utilities.
2.Open Directory Utility.
3.Click the lock in the Directory Utility window.
4.Enter an administrator account name and password, then click OK.
5.Choose Enable Root User from the Edit menu.
6.Enter the root password you wish to use in both the Password and Verify fields, then click OK.
Note: If you are troubleshooting an issue that prevents you from logging in as an administrator, follow the steps in this article to enable the root user.
Mac OS X v10.4.x or earlier
1.Click the Finder icon in the Dock.
2.From the Go menu, choose Applications.
3.Open the Utilities folder.
4.Open the NetInfo Manager utility.
5.Click the lock in the NetInfo Manager window.
6.Enter an administrator account name and password, then click OK.
7.For Mac OS X 10.2 and later, choose Enable Root User from the Security menu.
8.For Mac OS X 10.0 and 10.1, choose Security from the Domain menu, then Enable Root User from the submenu.
9.If you have not previously set a root password, an alert box may appear that says “NetInfo Error,” indicating that the password is blank. Click OK.
10.Enter the root password you wish to use and click Set.
11.Enter the password again for verification and click Verify.
12.Click the lock again to prevent changes.
Source : http://support.apple.com/kb/HT1528