I can't find that group thing, either, but the ppa repositories for KDE 4 are here:
deb http://ppa.launchpad.net/kubuntu-members-kde4/ppa/ubuntu intrepid main
deb-src http://ppa.launchpad.net/kubuntu-members-kde4/ppa/ubuntu intrepid main
So in synaptic, go through settings -> repositories like you have and click new, then put in the appropriate lines (so "intrepid" in the distribution line, "main" in the sections line, and the uri in the uri line).
Then go along to this place
and follow the instructions for authenticating the ppa. Those instructions can be accessed by clicking on the blue question mark next to the signing key. The command should be
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys CA967634
but it's best to check with the site first in case the key has changed since this post.
Don't forget to reload synaptic. You should then be able to download the packages you want.
Once KDE is installed, you should be able to just log out from GNOME, and then find the "sessions" or "options" menu or whatever and pick KDE from the menu.
Hope it works for you.