To access a shared folder that isn't one of the folders listed on the File | Open | Other User's Folder dialog, the mailbox owner needs to grant Reviewer permission to the root of the mailbox, as well as appropriate permission on the folder.

The user who needs access then goes into Tools | Services, brings up the properties for the Exchange Server service, and on the Advanced tab, adds the mailbox.

After that, the user will see in the folder list all those folders in the secondary mailbox to which he/she has access and can add any of those folders to the Outlook Bar. See http://www.slipstick.com/outlook/links.htm#addtobar.