In Outlook 2011 for Mac, select the Tools menu and then Accounts.
In the window that appears, select your Exchange account and click the Advanced… button.
Click the Delegate tab. In the section named “People I am a delegate for:“, click the Add button.
The Select User window will appear. Type in the name of the shared mailbox in the text box and click the Find button. Select the desired user from the search result list that appears and click OK.
Click OK to close the accounts window. After a brief period, the shared mailbox will appear as a folder in the View list on the left side.