In an Exchange mailbox? 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 | E-mail Accounts or Tools |
Services (depending on the Outlook version), brings up the properties for
the Exchange Server service, and on the Advanced tab, adds the mailbox.

For more information on folder permissions, with how-to screen shots, see
http://www.howto-outlook.com/howto/permissions.htm and
http://office.microsoft.com/assistance/preview.aspx?AssetID=HA011134811033