Hello WernerF and welcome to these forums. You'll find many helpful people here and a lot of useful information. Make sure that you get the excellent User Guide too:
http://www.cockos.com/reaper/userguide.php
Ah, when we mention "Input Monitoring" here we generally mean the button on the Reaper Track Control Panel (TCP) - it is the speaker button you can see on track #7 here (you can probably ignore the second annotation, that was for selecting the input device):
Sending from a track to the Master is enabled by default, so you should see the track meter and the Master track meter move when Reaper receives the audio. Is that the case?
Have a look at section 3.5 in the User Guide.
Also, can you please post screenshots of the Audio device config is Reaper's Preferences?
How to describe your hardware set-up (in Post #3)