Mouse sensor + CX-20

I’ve got a CX-20. I would like to attach a mouse sensor.

The above picture is of a CX-20 board. It is the same kind of board as I have. The lower picture is taken from the arducopter tutorial for attaching mouse sensors to a APM. According to the tutorial a connection to A3 is necessary. But on the CX-20 board, A3 is unavailable.