Thank you very much for your response: it helped point me and to persist. Watched the video to finally find the 3rd level File Transfer option under the default Charging for USB use. Two trips to Verizon store did not get me there - last told me new phone was just not capable. Wife could not find it, we had hours into settings, different cables, etc. All other phones we have had automatically connected when plugged in, no switch required.
All other phones we have had automatically connected when plugged in, no switch required.
That's one of the changes with the Marshmallow operating system ... the USB default is charging only.