You need to burn the iso image to a cd, e.g by using k3b (tools -> burn cd image) or by doing as root
Code:
#cdrecord -v dev=/dev/hdc isoname.iso
Change the dev=/dev/hdc to that of your cd writer and isoname.iso with the name of the kubuntu iso. Once you have done that, reboot and run the installation from the cd you created.