I prefer MIDI because of USB grounding noise issues, but your interface doesn't have MIDI input. So you'll need to connect the drums to your PC with USB cable. I'm assuming you are using Windows. Once connected, turn the drums on. The PC should recognize them.
Open Reaper and go here. You should see the name of your drums. If the drums don't say "Enable" under input devices, right-click them and turn them on.
Create a new track in Reaper. Set the input of the track to the "All Channels" option of your MIDI drums. Record enable the track and hit the drums. Do you hear them playing in Reaper and see MIDI input on the track? If so, you are good to go. Hit record and record them as MIDI.