If you use the throttle channel to activate your motor in one of the glider presets, you'll have problems with airbrake/butterfly, since that operates off of the throttle stick. Your best bet would be to operate the throttle off one of the aux channels, and set that aux channel to the left-hand slider. You do that in the AUX menu.

To operate the throttle with a switch, use a free PMIX and select offset as the master, and the channel operating the throttle as the slave. Just adjust it so that when the mix is "ON", the motor's turned off. When the mix is "OFF", the slider will control the throttle.