Hi
I use the MKS 50 panel (to control my Juno 2) in reaper and I have it set up like this
Reaper
Options – Preferences – MIDI devices enabled in and out
New track with CTRLR as a plugin – Hardware output set to whatever my midi device is called
Settings in CTRLR plugin
Midi
Input from plugin host checked
Output to plugin host checked
Hope this makes sense