The way you'll program MIDI with Resolume Arena is after you enable your MIDI Device under Preferences>MIDI, go to the menu [Mapping] where you'll find two options 1) Edit Application MIDI Map or 2) Edit Composition MIDI map then choose what you want to map to MIDI either in Layer or Deck Focus and hit the key on your APC.
You can also check P106 of the R5 manual for more information which is always a good place to start
I am thinking about buying this controller. Are you stillusing APC Key25 with Resolume? Would you mind sharing some of your experience? I am curios how you mapped the piano keys. Thanks in advance!