Thanks for the suggestions.
I searched both suggestions, downloaded and unzipped Twainable.zip plug-in in the Effects directory, restarted Paint.net and... no change.
First, my copy of paint.net has no "Tools" in its "Effects" menu. So I don't see how that's going to work. I checked for Paint.net updates to see if I'd somehow downloaded an old version of Paint.net with an old menu or something. No updates. :?
It did dawn on me that the Sonix camera found by Paint.net is the built-in webcam in my laptop (that I don't use). So that was irrelevant. :oops:
I can't see where I've failed to follow any instruction?
Why isn't the plug-in designed to display in File=>Acquire=>From Scanners or Camera as the menu implies and the camera does? I don't fathom the design logic.
So far, it appears that Paint.net won't run my CanoScan 8400F? :?: