I'm not sure what you mean by "next channel"/"previous channel", but I'll assume that's something inside a sim.
The hardware side of this should be simple enough. Connect the middle pin of the switch to one of the GND(negative) pins on the Pro Shift, as shown here:

It doesn't really matter which one as the board has a common GND, but for wire length I recommend using the GND pins in the "SWITCH" segment.
Now you can connect the remaining two pins on the switch to two of the positive pins in the "SWITCH" segment.
The pin number doesn't really matter, just keep in mind most switches have the bottom pin on when the switch is up, and the upper pin on when the switch is down.