An ISO is a CD image - you will need to burn it to CD first. Make sure to burn it correctly, you should see a set of directories and files, not a single file on the CD. In K3B, you can burn a CD image by going "Tools" - "Burn CD Image". Nero is similar.
Put the CD in and reboot, and you should get the Kubuntu desktop as a live CD (assuming you have the latest version "6.06 Dapper"). On the desktop is a link that says "Install Kubuntu" or something similar. Run that program, and you should be able to install Kubuntu.
I hope this helps