You can find the Public folders setting via the Menu>Accounts section in the Exchange Web Services tab of the given account or via right-click on the specific account in the folder list.

• In the Public folders window or section click the Edit... button next to "Choose which public folders to show:"
• A list of domain shared calendars and/or folders should be displayed.
• Select the ones you want to add, then click the Ok button. 
• Then, click Save & Close on the Accounts window or close the Public folders window.