Use this method if you are trying to gain access shared mailbox in the Outlook app for Mac. 

To open the shared mailbox:

On the Tools menu, click Accounts:

When the accounts window opens, select "Advanced" in the lower right hand corner:

While on the "Delegates" tab select the small + sign under "Open these additional mailboxes"

This will bring up a window that will allow you to find the mailbox you wish to add. Search for the shared mailbox in the search bar, select it, and then select "Add" to finish adding the mailbox.

The mailbox should then appear on your main Outlook screen.

Please note that you must have permissions to use the shared mailbox in order for this process to work.