type in the name root anyways and follow with root's password. When the gui loads up, go to configure my desktop > system > login manager > users, and uncheck root from "hidden" users.
That said, you DON'T log in as root do you?
In all seriousness, I try to strongly disuade people from logging in as root. Much better to just flip to console and "su" yourself into root if you need to. I can think of a million reasons why you shouldn't log in as root, and really none other then "convenience" why you should. And since convenience and security are generally fighting with each other... (more of one means less of the other)