This might help.
https://youtu.be/uaFnnaTKI_Q
I have read however that the Komplete Kontrol hardware/software does not yet integrate well with Reaper. What that means is that controlling Reaper functions such as play, pause, record, looping etc may not be possible yet.
But that does not mean that the controller will not work with Reaper! It just means that those functions may not work at this time. You should still be able to browse your NKS instruments and control them from the keyboard.
My advice...watch that video, make sure you have installed the Komplete Kontrol software, and read the Komplete Kontrol manual.