I just had this problem and just figured it out.
Here are the steps:
- Login to Outlook Web Access (the web interface for your Exchange Server).
- Click on public folders (for me, it was all they way in the bottom left).
- The, navigate to the public folder of your choice (I needed a calendar).
- Right click on the public calendar and select copy from the context menu.
- Navigate back to your own mailbox (I clicked "mail" in the lower left).
- Right click on your own calendar and click paste (I added it to a new folder).
- Then, return to calendar.app and refresh your calendars (View menu -> refresh or command R).
- After doing that, the public calendar popped up along side my other calendars.
Hope it works as well for you as it just did for me!
Also, I think this question is a duplicate of this one.