In Outlook, go to the Tools Menu, then Accounts, The with your account selected, click on Advanced: In advanced options, click on Delegates Tab (between Server and Security). In the bottom half of that screen is the 'Open these additional mailboxes' section. These are where you can enter any shared mailboxes you have permission for. Click the + and enter the email address of the mailbox you have permission for. Click the correct mailbox, and select Add. It will prompt you again with the Permissions, check the checkbox and click Allow:
The shared mailbox will show up in your navigation pane under your mailbox.
