The solution is to have a helper GUI process or shell extension (loadable DLL module for Explorer) which will communicate with the service and either add the mounting point itself or broadcast the message when the mounting point is added or deleted.
Callback File System 2.0 and later includes such helper shell extension, CBFSMntNtf*.dll. Once installed, it catches notifications of the user-mode CBFS API and notifies Explorer and other applications by broadcasting the message. Your code specifies whether this helper DLL needs to be installed in parameters to Install() method.
Note that for the mounting point to be visible to the user's session, it must be created either as a global mounting point (without CBFS_SYMLINK_LOCAL flag) or it must be created for particular session. In case of "local" mounting points are used it is required to specify the AuthenticationId parameter during the AddMountingPoint call in order to create a local mounting point for another than the current logon session. The mount notifier helper DLL is required to be installed too. For additional information about local mounting points read "Mounting Points and Authentication IDs" topic in CBFS documentation.