I don't really understand why you are having so much trouble, normally none of this is needed.
Anyway, to answer your question, edit the /etc/group file by hand with your favorite text editor under KDE (like kate or nano), find the line starting with "mythtv:" and append "linuxmce" to it. So that it looks something like:
That should get you past the error I reckon.
PS. I didn't mean to try and "correct/contradict" you rwilson131, I completely overlooked your reply before, sorry.