The way I used to enable the super user was this. Go to Applications/Utilities and launch, "NetInfo Manager"
Under the "Domain" menu, there is a submenu called, "Security". You may first have to select the item there called, "Authenticate..." Just use your normal password.
Once you have authenticated (you might not need to do it), then select the item in the security menu called, "Enable Root User".
It will ask you for a password. After this, you will be able to "su" and use the password you specified.