Mapping plugin controls to external MIDI is pretty easy in Reaper...open the plugin window, and tweak some paramter that you want to map to external midi control--let's say Cutoff. Then click the "Param" button at the top of the plugin window, and at the very top of the drop down menu, grayed out, you should see "Last touched: [Cutoff]" If you click in that drop-down menu on "Learn," the MIDI learn window will open, and the very next MIDI message you send from MIDI Designer will be mapped to that param.