From the page you sent me to:
"Insert | Slides From File is now Insert | Add Slide | Reuse Slides.
Navigate to your file, and the slide thumbnails will show up in the task
pane. There's no way to add multiple slides at once, so be prepared to click,
click, click your fingers off. (Don't shoot the messenger; I think it's
ridiculous, too.) But you can insert all the slides by right-clicking and
choosing Insert All Slides."

Okay, this looks like it might have been a step in the right direction, but
where the instructions say "Instert | Add Slide | Reuse Slide" is not what I
see. When I click on the Insert tab in the Ribbon, I see grayed out choices
for "Picture", "Clip Art", "Shapes", "SmartArt" and "Chart".  The only
selection not grayed-out is "Photo Album".  I would assume "Chart" is what I
need, but that's not available while my master file is open and ready for
other slides to be inserted. I've not found any other Insert menu anywhere,
so I'm still looking for the solution.  Thanks.

YES!  This is exactly what I needed. I completely missed seeing a dropdown
on the New Slide button.  

Thank you!