My problem was with the GT. I hadn't gone to Wireless & Networks and configured USB properly.
Here's the "Help" file I generated for my wife. Hope if works for someone else.
GT Select Menu – Settings – Wireless and Network – USB Settings
GT Select Mass storage if not already selected
GT Connect the GT to the PC using charging/USB cable
GT A new screen appears USB Connected with option to Mount
GT Select Mount [GT cycles through several steps and may present option to mount a second time. If it does select mount again.
PC PC should display one or two “auto play” windows (one if you don't have an SD card. Two if you do) – both of these windows may be closed
PC Open Calibre
PC On the menu bar select Connect/Share – Connect to Folder
PC Navigate to the folder on the GT that you wish to hold your eBooks from Calibre. [Hint – The folder will be on one of the two drives that opened when you selected mount on the GT. I used a folder called downloads on the GT internal memory]
PC Two new icons will appear on the Calibre menu bar (Send to Device and Device)
PC You may now manage books on your GT using Calibre.
Don’t forget to eject the device (GT) from the PC prior to removing it.