First thing you need to make sure you have universe source list under /etc/apt/sources.list file
Type in a terminal:
$ sudo apt-get update && sudo apt-get install kubuntu-desktop
During the installation process, you should be asked whether you want to use KDM or GDM as your default display manager. If you think you’ll use KDE more frequently, make KDM your default. If you think you’ll use Gnome more frequently, keep GDM as your default.
When KDE is done installing, log out. Click on Options and then Select Session.
In the Sessions dialogue, select KDE and then Change Session.