Yep, I am still getting my head around a lot of it.. there are example to be found panels in the Ctrlr app download, these are good for referencing examples of scripting / methods.
Atom has written a getting started guide here: http://ctrlr.org/getting-started/
I would recommend using the Ctrlr MIDI monitor found under the Windows Menu to see what your DX21 sends out in the form of CC or Sysex messages. You will also need the instruction manual for the DX21 which hopefully will have comprehensive tables showing you the MIDI CC and Sysex info.
Hope this helps : )