Everything in /etc belongs to root, so you will need an instance of gedit with root priviliges. Do the command with gksudo before it.
Originally Posted by schragge
gksudo gedit /etc/apt/sources.list
Are you using ctrl+shift+c and ctrl+shift+v ? You should also be able to selct "copy" and "paste" out of the edit menu or with a right click.
Originally Posted by Shallowmind
Another way that seems to work (I just tried it out here...) from the terminal to somewhere else and at least from this firefox window to the terminal is to highlight (click and run the mouse over) the text to be copied, and, leaving it highlighted, point to where it is to be copied to and click the middle mouse button (on my mouse, that is the wheel).