Try Synaptic or use sudo apt-get install kubuntu-restricted-extras in a
terminal. I don't like Packagekit myself. I am not sure why you can't
install the restricted-extras because I have used K/Ubuntu for years and
never had a problem in dozens of installations. I have both Kubuntu and
Ubuntu restricted-extras installed on several computers now.

When asking for help it is good to provide as much information, such as the
precise error message, in this case.