find the directory it is in and type:
and see what the response is.
if the su and p/w did not help, try adding a ' -' without the ' ' marks after su
then enter the root password then try to edit the file again. what are you using to edit the file? are you doing this via the GUI, if so you will need to login to your GUI as ROOT, not as your normal user.
open up a terminal, su - over to root, then open your editor from the terminal
ray@media:~$ su -
media:~# vi /etc/apt/sources.list
that is the commands i use to open and edit my apt-get list. for yum it would be just entering the correct path to your .conf file and it will work.
you can replace the vi with gedit or putty, or what ever you use as your editor. i use vi/vim as they are the only editors i know from the CLI, and gedit when i am touching the box and in GUI mode.