OK, I've dug my eeePC out of storage and think I've got it figured out. Before you do the "sudo echo" command, first type
sudo touch /etc/apt/sources.list
then
sudo chmod ugo+rw /etc/apt/sources.list
and then carry on with the "sudo echo" command as before.
What that does is create a zero length empty file and make it writable by everybody. Once you've gone through the rest of the instructions and moved the original file back into place it will go back to having more restrictive permissions again.
Give that a go and see how you get on.
Andrew