Cateye Adventure Wireless Speedo with Altimeter

Somehow I lost the link from the transmitter unit to the head. Now I find that I have at least two transmitter units and neither of them connect to the head unit. I vaguely remember that there was a process you had to go through to mate the transmitter to the head unit but can't find the manual now and can't find anything on the web.

Does anyone know or remember how this is done?