If you have paid for the Online Update service, I don't know how to answer your question.
If, like most people, you can't/won't pay for Mandriva's automatic Online Updates, then turn off the Online Update System in your system tray, and visit
http://easyurpmi.zarb.org/. Follow the instructions on that page. Select *at least* an Updates repository. (I would recommend Main, Contrib, and both PLFs as well). When you get to step 3, copy the information it presents to you, and paste it into a command line as root, and wait while it finishes.
Once you have your repositories set up, go to the Mandriva Control Center (Star -> System -> Configuration -> Configure Your Computer). By default it opens on the initial rpmdrake section. Select the option to Update your system (Lower left choice, on my system). Answer "yes" when it asks if it is okay to go online to search for updates. When it returns a list of available updates, pick which ones you want to install, and click "Install". (You can also fine-tune your updates, choosing Security fixes, bug fixes, and routine updates, in any combination.)
It's not automatic, but it's not hard, either.
