It finally came to light (picture light bulb overhead suddenly lighting up).
I had been looking for an "apply" button to activate the selected effects (like WavePad), but could not find any. It never occured to me that the selected effects become activated, in real time, on the selected track.
My next MixPad mystery to sort out (maybe), is that the edited files exported from MixPad come out sounding "mushy", not quite as crisp as the original tracks from WavePad.
Anyway, many thanks for answering, much appreciated.