The name of the root-user is "root" so you can't give it any other name. Try typing "root" (in lower case) as the username and then use the password you specified during install. Or click root in the list on the left and type only the password if you'are using the kdm login manager. Also remember that linux is case-sensitive so you need to type the password and usernames exactly the same way as during the install.

If you for some reason can't remember the root password you probably will need to do a reinstall.