When I try to install software updates using the "Software Update" GUI, I am asked for the root password. Although, I can 'su' in a terminal, the password is not recognized as correct in the GUI prompt. I believe this is due to the user account name being different from the root account name. How can I still use the GUI then?
AFAIK, you need to use your own password, and be in the sudoers file. Can you run "sudo su" form a terminal? If you can't, that would be an indicator that you're not in the sudoers file, hence the password rejection.
Make sure you're listed in /etc/sudoers, or run man sudoers if you're not sure of the syntax.