How to address the I/O via pd?

Hmmm you cannot do that directly from PureData at the moment (though it could be implemented in an external in the future).

We would normally do this using libpd or Heavy and a custom render.cpp file. You will have to read your sensors and then sending them as messages to the patch using libpd's libpd_float() and/or libpd_start_message() or Heavy's hv_vscheduleMessageForReceiver.

You can find an example of interfacing to the MPR121 in examples/06-sensors/capacitive-touch and an example of custom render file in examples/08-PureData/customRender.