Here are easiest steps to enable Root User on Mac OS X Snow Leopard and later version of OS X like Lion.

Step 1: Go to System Preferences

Step 2: Select Users & Groups under System Tab

Users & Groups under System Tab

Step 3: Select Login Options

Step 4: Click Join next to Network Account Server. It will open Directory Utility dialog box

DirectoryUtility

Step 5: Click Edit Menu from the Menu Bar and click Enable Root User. System will ask you to enter password and it will enable Root User.

Read More at https://support.apple.com/kb/HT1528