Comment 20 for bug 375593

When trying to edit OS configuration files, you need to have root privileges. The command to use in this case is:
gksu gedit /etc/samba/smb.conf

It will ask you for your password, just use the same password you use when you log into Ubuntu.

For most help on these types of questions, you should join the ubuntu forums at ubuntuforums.org
You will find a lot of very helpful people there.