If you want to use Tens pad of Central, you can connect Tens cable and Tens pad first. Then you need connect the TENS cable into the 3.5mm Audio to BNC cable, and converts the Tens cable to a 3.5mm MP3 style plug. And plug the adapter cable straight in the socket of Contact on Central.
The detailed as below:
The picture of 3.5mm Audio to BNC cable as below: