With regards to connectivity with Reaper using these two steps;
Please check that both input and output sessions (named: Session 1 or DAW from your Audio MIDI Settings) are enabled in;
• Reaper Preferences > Audio > Midi Devices
• Reaper Preferences > Plug-ins > Control/OSC/Web
Here you need to add a Mackie Control Universal, control surface. Then choose MIDI Input and MIDI Output as your previously enabled session in step 1, Session 1 or DAW. If both Input and outputs are the same it should work.