On EnumerateOutputDevices, it reads:
To open the device you want, simply pass the device in to the appropriate WASAPI class depending on if you are playing back or recording...
var outputDevice = new WasapiOut(mmDevice, ...);
var recordingDevice = new WasapiIn(captureDevice, ...);
var loopbackCapture = new WasapiLoopbackCapture(loopbackDevice);
But as of version 2.2.1, there doesn't seem to be a WasapiIn class.
Edit: I guess the new class is WasapiCapture