You have to install the unrar rpm package. IIRC it doesn't come on the cd but is available from PLF. If you are not familiar with setting up urpmi sources, go here and follow the directions:
http://mandrivausers.org/easyurpmi/index.php
Most mandriva users set up main, contrib, plf-free and plf-nonfree sources. This greatly increases the amount of software available and simplifies installation. Once you set up these repos, the unrar rpm will be available along with a ton of other stuff.
