Usually, shared mailboxes simply appear in the account/folder list in Outlook, but sometimes they're just stubborn. If that's the case, then follow the instructions below to add it to Outlook.

Step One

Select the File Menu, and choose Account Settings from the Account Settings button (the square one).

Step Two

From the list of accounts, select the account that the shared mailbox is attached to, and press the Change button above the list.

Step Three

Click/Tap on the More Settings button (bottom left)

Step Four

Change to the Advanced tab, and then press the Add button next to the additional mailboxes list.

Step Four

Enter the email address of the shared mailbox, then press OK.

Instructions for this can be found on the Microsoft Support site - Open a Shared Mailbox, Calendar or People folder in Outlook for Mac.