Thanks @sloopsailer
I haven't sailed on Princess for a few years but booked a cruise yesterday. One thing I discovered PDQ, is that I needed to delete the version of the Medallion app I had on my tablet and reinstall.... the app had not been updating as is usually the case.
It worked great and, like you say, pretty straight-forward to get my wife and I in the Green lane along with dinjng reservations and a few other things I needed to take care of due to not choosing the 'all included' option for our reservation. We don't drink so an a-la-carte approach to covering soft drinks, gratuities and wi-fi ended up saving us about 800 USD for a 15 day cruise.
Also, the PC worked fine too, but I thought I'd give the app a go mainly out of curiousity.⁹