That's an odd one... any chance that your phone's SD card might be write protected somehow? Do you find that when you change an item in Pleco's settings screen it doesn't "stick"? It seems like for some reason the flag that our app sets to indicate that you've tried out OCR (which it does as soon as you open the OCR screen) isn't remaining set when you switch into Add-ons.
One easy alternative would be to buy the add-on directly from our
website instead - avoids the "not tried" check entirely - but since this is such odd behavior, it might be better if we get to the bottom of it first so that we can be confident it won't cause you any other problems in the future.