MCC->Hardware->Display Manager Chooser
i admit putting it under hardware is a rather odd choice, but whatever, it's there :-)
i think most people are complaing about the KDM looking ugly because it's actually mdkKDM, you can switch to the regular KDM in the same spot mentioned above. (I don't use graphical login anymore, so I don't know for sure.)
as for the error, i get the same thing when starting artswrapper from a console as a regular user.