Access a Shared Mailbox in Outlook Desktop Application

Outlook for the PC will automatically mount a Shared Mailbox after the next synchronization of the Outlook Address List, typically within 24 hours.

If that didn't work, then manually add the shared mailbox to Outlook:

1. Open Outlook
2. Choose the File tab in the ribbon
3. Choose  Account Settings, then select Account Settings from the menu
4. Select the Email
5. Make sure the correct account is highlighted, then choose Change 
6. Choose More Settings > Advanced > Add
7. Type the shared email address, such as info@contoso.com
8. Choose OK > OK
9. Choose Next > Finish > Close

More information is available at Microsoft - Open and use a Shared mailbox in Outlook

Accessing a Shared Mailbox on Mac:

1. In the top of Outlook, select the Tools tab
2. Click Accounts > Advanced > Delegates

   

3. Add the mailbox under Open these additional mailboxes

   

4. Under "People I am a delegate for:", click the plus button
5. At the "Choose a Person" prompt, search for the shared Mailbox

   

6. Click "Add" then "OK"

   

7. Close the Accounts window.
8. Click the "Send and Receive" button in the top right corner.

   

9. The Shared Mailbox will be added as an additional Mailbox with its own Inbox.