Enabling the root user in Leopard is quite easy. To enable the root user account do the following:

  1. Open Directory Utility located in Applications/Utilities.
  2. Unlock Directory Utility by clicking the the lock icon and entering your password.
  3. Once it's unlocked go Edit » Enable Root User and type in a password, make sure it is secure and memorable.

You have now enabled the root user account in Leopard. A word of caution: Do not use this account as your normal account. It is very dangerous to run as the root user. You can easily render your Mac inoperable if your are mucking about in the file system.