On a trip to the US, I bought a Behringer FCB1010 MIDI foot controller. MIDI is a sort of networking protocol for musical instruments. When connected to a device that understands MIDI, a MIDI foot controller lets a guitarist send signals to change effects, adjust the volume, sweep a Wah effect up and down, or adjust an effect level. On the Mac it represents a programmable stomp box for playing and recording.
This controller seemed both cheap compared to alternatives and also fairly rugged. Yet two things were wrong out of the box. First, the controller came set up to plug into a US/Canada wall outlet with no adapter and no way to select voltage. So for $9 I got a transformer.
Second, neither expression pedal sent any signal out of the box. Since I had no idea what I was doing, I ended up installing the demo version of iFCB first including the package that let me see the FCB1010 in the MIDI setup. Not sure whether that was necessary (though I probably will go back to pay for iFCB once I better understand what I’m doing and want to get the FCB1010 to do more). Problem symptoms included getting nowhere trying to have MainStage learn the settings from either pedal, seeing MIDI signals in the status box at the top of the MainStage window for the switch pedals but not for the expression pedals, and seeing the same failure in more detail with MIDI Monitor.
Turns out the second problem was even easier to fix than the first, once I stopped trying to find the answer (or even anything I could comprehend) in the documentation. Instead I followed this procedure to recalibrate the FCB1010 expression pedals. With recalibration done, I turned the FCB1010 power off and on, restarted MainStage just for good measure, and found that MainStage had no trouble learning the settings.