this hasn't worked for me.
When i follow the additional steps using terminal, the second command brings back the same outcome - 'David is not in the sudoers file. this incident will be reported'.
I've been having trouble with login on for a week but didn't realise this was the problem. I've reset passwords, reinstalled OSX (twice) and now, bizarrely, i can log onto the mac mini as the administrator, but once logged in, the 'administrator' password doesnt work for anything. I can run Software Update because it comes up with a message:
"Software Update wants to make changes. Type an administrator name and password to allow this".
I then put in my password but it shakes and tells me its wrong.
Can anyone advise any other solution? I'm kind of stuck - because the system doesn't recognise my admin password for updates, i cant update anything. And also because my itunes library was set up by a newwer version (I reinstalled OSX believing it would fix it reverting it back to a previous version) i can't listen to any music.