https://www.youtube.com/watch?v=loz5BJVpBII
This shows all settings of the iD software but basically it's as simple as opening it and turning up the fader for the channel you're recording.
(Mic 1 or 2).
Mk1 and Mk2 are going to work the same.
Then just be sure record monitor in Reaper is off.
But to be clear....this works for audio input only. Direct monitor can't help with soft synth playback, for that, you need to set lower buffer size and make sure you've not got high latency plugins loaded in reaper that will add to the system latency.