Outlook Desktop Application

Select File, Info, Account Settings, Account Settings...
Select the Data Files tab. Select the User then select Settings...
Select the Advanced tab. Under "Open these additional mailboxes:" you will see a list of shared mailboxes the user has added. Select Add.
Type the name of the mailbox you want to add, and select OK.
Select OK and Close to return to Outlook. The mailbox will appear on the left-hand menu of Outlook.

Outlook for Mac

Use this method if you want to view and manage the email for the shared mailbox in your Outlook application for Mac computers.

On the Tools menu, click Accounts.
Click the Plus Sign to add an additional Exchange account.
In the E-mail address field, enter the email address of the shared mailbox and click Continue.
On the next screen which prompts for a password, select "Sign into another account." Enter your MBU email address and click Next.
On the Enter password screen, enter your MBU password and click Sign In.
If your credentials are correct, you will see a success message, and the shared mailbox will show as a separate account in your Outlook application

Tip: If you mistype the email address of the mailbox, the result is "No match was found." Try retyping the email address.

If you're trying to add a mailbox that was recently created and it doesn't show up in the directory, follow the steps below.