If you mean to log in to you X session as root, you really shouldn't do it - there's no valid reason for it.
If you mean to temporarily log in as root from the command line to do some admin stuff, just type:
+ Enter and your root password.
I don't know that distro. if there's no root password enabled by default, then try:
and type a USER password. It'll ask you to type the new root password (+ confirm the root password).
Afterwards, you'll be able to log in with 'su'.