The Akai APC40 is a great controller for MIDI mapping to applications (Traktor, etc.) as it has a large and varied set of controls available, along with multi-coloured pads. For a MIDI application to take full advantage of these the APC40 needs to be in Firmware Mode 1; upon power-up, however, the APC40 enters Firmware Mode 0. Switching to Firmware Mode 1 can be done by opening Ableton Live, waiting for the APC40 to be recognised as a control surface (assuming it is configured as one), and then closing Live. apc40-mode is simple utility that will let you switch between the three firmware modes at the command line, so you don’t have to open Live (or even have it installed).