fstab is a system configuration file, you need higher priveledges than a standard user, in most distros this would be the root (super user) account that can edit ANYTHING in the system, ubuntu by default uses sudo (which i dont like, but i use sudo all day on arch :S).
Open up a console (aterm,konsole,xterm,gnome-terminal, whatever) and try
joe is an editor, make your changes and then ctrl-K ctrl-X to save and then quit, there are other editors like nano or vi (someone else will have to advise how to use that) or you can type in a console
Code:
sudo gedit/kate/whatever_GUI_editor
then open the file like that.