From Outlook, Click on the little folder icon located near the bottom left.
This will change the view on the left to show much more information.
Expand the folder "Public Folders - firstname.lastname@example.org"
Expand "All Public Folders"
Expand "tbh.com Public Folders"
You will now see a list of all Calendars setup in the Public Folder space.
Right click on the Calendar you wish to add.
Select the option "Add to Favorites..."
You will get a pop-up window. Click "Add".
Now go to your regular Calendar section in Outlook.
Under "Other Calendars" you will see the new Calendar available.