How to view public Exchange calendars from iCal?
The above doesnt' REALLY work for people, especially not with Outlook 365. Here's the way that worked for me:


With iCal open go to the menu bar at the top of the screen and select iCal, it's to the right of the Apple Symbol in the upper left corner.
Select Preferences
Once the Preferences window comes up select the Accounts section then select the Exchange account to bring up its information.
Now select the Delegation tab.
Under the list is a + sign. Click this to add a calendar.
Begin typing the user's name as above and the auto-completion will begin. Once you've found the correct user's name select it to add it under the Users column.
Under Accounts I can access: you will now see that user's name. You can enable or disable their calendar from displaying by checking or unchecking the boxes under the Show header.


I found these instructions from the USC website.