Adding Login Items to a Guest Account

Using 10.7.4 on my local home computer, I've been following these instructions for 10.6 on adding login items to a Guest Account:

"Download and install the Snow Leopard Server Admin Tools from Apple. Then open Workgroup Manager. Log in to the address localhost, and authenticate with an admin's username/password. Under the View menu, choose Show System Records. This will show the Guest account in the user list in the sidebar. Click the Guest account in the sidebar, click Preferences in the toolbar, and then click Login -> Items to the right."

YES... I did download and install the LION Server Admin Tools 🙂

It all works, except that the options to make changes are always greyed-out and it says '/Local/Default Not Authenticated' even though I've typed in my administrator username and password. Clicking the 'padlock' brings-up the authentication screen again, but my administrator username and password just cause an error message saying "The server failed to accept the login information you provided".

Help?