1. Login to the Admin account (not the normal account)
2. Open up a command shell in the Terminal application by selecting - Macintosh HD -> Applications -> Utilities -> Terminal

At the command prompt type this command:
sudo passwd root

You will then get the following prompts.
Enter Password:
Changing password for root
New password:
Verify password:

The first password you are asked for is the password for the admin account, to prove that you are authorized to make changes to this system. After that, you enter the new root password (twice, for verification).

That is all!
The "root" account is now enabled, with your chosen password.