Stu & KMan,
After a number of guesses, followed by much help from Michel, we* have everything working. Here is what took place:
I registered ISY Portal with my email address. My wife got an early relese Echo, and she set it up right away using her email. Thinking that I'd keep things simple, I used the same email address I used for ISY Portal, for Amazon, as it is connected to my wife's Amazon Prime account.
Everything seemed good at the start. I did enter my spoken words on the ISY Portal and when I did the Discovery, they showed up in the list as expected. However immediately after the discovery completed, Alexa announced that she didn't find any devices. A bit contradictory to what was on the screen. Long story short, you can't use the Amazon Prime account association, as it does not fully complete the discovery process.
After changing to connect to Amazon with my wife's email, everything worked perfectly. I've added about 15 devices & scenes so far, and it has been flawless The only downside of this, for me, is that I will need my wife to log in on Amazon whenever I need to discover new devices. I'm fine with that as its not something I'll need to do often.
(* we = pretty much 100% Michel)