Configure is the same as Backup and Extras. I can see it but it's not highlighted or clickable. The other two options - Manage and Support work fine.
Have you tried re-installing Galaxy? Sounds like there's something wrong with it.
If all else fails, download & install the DLC manually from your library: https://www.gog.com/account
Click "Witcher 3: Wild Hunt, The" DLC Downloads will be listed towards the bottom.
Thanks for your help . As with all Witcher/Proj Red products the bugs were on top of bugs. After uninstalling and reinstalling Galaxy It opened up enough functionality to at least get the DLC downloaded (after several attempts) and operating in game (after a couple attempts.)
The game is graphically awesome and the story is solid and engaguing, but the user interface and game controls (as with EVERY witcher release) are infuriatingly burdensome. My guess is the developers never actually played their game, only sat back at the monitor and admired it.