This tutorial shows you how to load Scaler in Reaper and control an external instrument:
To control external instruments within Reaper with Scaler 2:
1. Open Reaper and create two new tracks.
2. Load Scaler 2 on the first track
3. Then load the instrument you wish to control on the second track.
4. Set the MIDI output of the Scaler 2 track to your external synth: