You need to update your repositories. Click on the icon "configure your computer". Type in the root password. Click on "configure media sources for install and update". In the top left corner, click on "File". Click on "Add a custom media". On "type of media", choose "FTP server". In media name, add whatever name you want, such as KDE4.2.1. In URL, add a media url, such as this for a 586 system, or this for an x86_64 system. Then click the box "Tag this medium as an update medium". Click "OK", and you can then update.