Bingo, Organicactus and rigdoctor!
In browsing youtube for answers, I had the impression that the white button was meant as a button, to be pushed DOWN. No wonder it had no effect when pushing down. By hooking a thumb nail on the lower lip of the button, I WAS able to slide it backward, toward the spool, and load the filament. All Bambu filaments are now recognized for color and type.
Told you it would be a Duh! user error. 
Thank you, one and all. As they say, even old dogs…