trying to add KDE to installed ubuntu

[Russ]

> You need to install KDE for Kubuntu, as you have only GNOME now. Do:
>
> apt-get install kubuntu-desktop
I open up a terminal and type 'sudo apt-get install kubuntu-desktop', it 
then asks me for my password, then it says it cannot find the package.

Any ideas please

Thanks
Russ