I did some reading before I bought mine, and I think I found this information on www.gossen-photo.de - but I can’t find any reference to the adaptor anymore.

Anyway, it is a small plastic gizmo that holds the batteries and fits into the battery chamber. I think that the best bet is to contact Gossen directly and inquire if they are still available. Take a look at http://www.gossen-photo.de/english/foto_service.html