You may not have permissions to view that folder. A Public folder owner can set the default permissions to not make the folder visible except to those who have permission to view them. Are you the Exchange Admin? If so you can view all folders through the tool PFDAVAdmin and see what the permissions are set to on those folders using that tool. Install it on your pc, not the exchange server. If you are not an exchange admin then you simply do not have permissions to view that folder. You would need to request permissions to view the folder through your IT department or the folder owner.