Open Outlook and click on the File tab in the upper-left hand corner.  

Click on the Account Settings button and select Account Settings...  

Under the email tab, click on the Change button.  

In the lower-right hand corner of the window, click on the More Settings... button.  

Under the Advanced tab, click on the Add button in the section marked Mailboxes.  

The Add Mailbox window will appear. Enter in the name of the shared mailbox and click OK.  

This will take you back to the advanced tab. Click on the Apply button.  

It will take a moment for the mailbox to be added. Once the process has finished, click on the Ok button.  

This will take you back to the change email screen. Click on Next button in the bottom of the screen.  

The next window will confirm that everything has been added correctly. Click on the Finish button.  

Close the Account Settings Window.  

You will now see your shared mailbox on the left hand side of the screen.