If you've successfully installed the distro, the ISO is no longer required. You can use Easy Urpmi to assign repositories, which you use for updates and extra software.
Since 2009, you can also set up repositories within the Mandriva Control Centre. You will find documentation online here - try the Mandriva tools guide.
To access the MCC from the main menu, use Tools - System Tools - Configure your Computer, entering your root password when prompted. The Software Media Manager should help you to set up the necessary sources.