It looks like you have the mandrake 9.1 default set.
If you want to do command line stuff, then just open a terminal and type the "su" command then put in your root password (as mentioned in the others posts) - you will see the change in the prompt.
If on the other hand, you want to do stuff in gui mode, then you will probably have the default set up where root is a hidden user - not a problem, but to "unhide" root for graphic login,
log in as your user, and presuming that you are using KDE for your desktop, then "K"menu>configuration>control centre (kde control centre, not mandrake control centre)
once the control centre is open, system>login manager. To unhide root, at the bottom of the login manager dialogue click on the "administrator mode" button, when it has finished loading, un check the box for root being a hidden user (you should only then have "nobody" left checked as a hidden user I presume that you could delete nobody if you want, but I havent tried that!) then close the control centre.
You should now have root showing as a login option when you are logging in or changing login from the "K" menu.