Go into the System Setup menu for the model, down to Channel Assign, then to to the next page (Next off of "Rx Port Assignments" page), which brings you to "Channel Input Config" screen. Cursor over to the AUX2 channel select it then flip the switch you want to control it.
Are you sure you want to use the flight mode switch, which usually controls normal, idle up 1, and idle up 2? If you assign it here you will be controlling both flight mode and AUX2 with it. I ask because by controlling both with a single switch you may well end up confusing yourself. I use m flight mode to control which governed headspeed I want to run (and what pitch curve is associated with it). I used switch D for the AUX2 switch, allowing selection of modes in the Ikon independent of what flight mode I'm in. Once you have settled on the ikon modes you want to run in each flight mode, then I could see combining them to a single switch. Maybe you're already there?
Also, FYI, I had to reverse the AUX2 channel in Servo Setup screen to make it work in the right "direction" in the Ikon software.