The website that you posted a link to contains a nice demo of what the sources.list file should look like. I'm not sure why mepis doesn't want you to edit the file directly. Maybe there is a gui utility to edit it in mepis. I don't know. In debian we do it by hand. We do everything by hand. Anyway, you can make a backup copy of your sources.list. Maybe name it sources.list.bak
cp sources.list sources.list.bak
then try to edit it with your favorite text editor removing the part that is causing the problem and adding whatever you want to add. This one in particular sounds good:
I am pretty sure you can use stuff from the debian repositories also, and the mplayer repository is useful if you want mplayer.
You may get better help if you post in the mepis forum or one of the websites dedicated to mepis users (ie www.mepislovers.org
). Or if those are no good for you, you can try the debian forum. It is awesome and after all, mepis is basically debian with a pretty face.