The First Username typed in the wizard gets admin rights, the others do not. Do you remember the username used as the first entry in during installation? That is the user with rights to edit the configuration.
Here is the link that may help as well. I was not aware the username and password were the same and used this link to reset. Perhaps a SQL query will reveal the users you created on initial launch.http://plutohome.com/wiki/index.php/Reset_the_password_for_the_Pluto_Admin_site
Or within the Plutoadmin, can you see the users and their rights?