To edit files outside of your home (and I think /opt) directory you need root privileges. If you would like to edit this particular file using gedit as root, type
into a terminal. You will be asked to type in your password in a GUI box, after which you will be able to edit and save the file.
gksudo gedit /etc/icecast2/icecast.xml
I don't see anywhere on that page instructions on changing the permissions of the file, but if you do need to do that (think carefully before you make such changes) then see the manual page and this tutorial on the chmod program.