We would suggest you please try the following steps. Make sure permissions are granted for the account in Active Directory.
Also please refer to the following technote :-
Title:- How to enable support for "Anonymous" when using VERITAS Backup Exec (tm) 8.6 and VERITAS Backup Exec (tm) 9.x for Windows Servers
http://seer.support.veritas.com/docs/238618.htm
Also please check permissions on the public folder and verify that the account being used in Backup Exec to attach to the public folders has rights to the missing folder.
To proceed further troubleshooting the missing public folders, please let us know if you have used any migration utility to get/transfer few of the public folders from Outlook, or from any other exchange version that you might be using earlier.
Also as a test purpose, we would appreciate if you could create 2/3 more new public folder's and let us know if you are able to view them in Backup Selection tab as well as in Outlook express.
We hope this helps.