You can't use SBUS directly on APM. You will need signal converter to use SBUS on APM, which I found got a little delay so it's not recommended. If you wanted to use SBUS, better throw away your APM and changed to Pixhawk.
APM only accept PWM & PPM signal. Lucky you, R12DSM can be used on PPM too. You only need to double push binding button on R12DSM till led changed to red which indicated worked on PPM.
Now you connect R12DSM pins (Signal, Vcc, Ground) to pin input channel 1 on APM board. But before that, you must activated PPM mode input on APM by jumped in signal pin 2 and 3 on APM board.
I don't have experience on KK2 but from what I know, KK2 can use PPM and SBUS. You only need to changed signal mode (PPM/SBus) on KK2 menu and connect receiver pin on Aileron pin input on KK2.