3 days later ...

After much hassle I was able to get my Restore Disks to work, restored my computer to factory setting, upgraded Windows 8.1 Pro to Windows 10 Pro, downloaded/loaded Sketchup and ran it, downloaded SketchUcationTools. In SketchUp Windows/Preferences/Extensions-install extensions, installed SketchUcationTools and up pops the Extension Store dialog. And when I run SketchUp the dialog pops up. At least I was able to disable it and it quits popping up.

You are not going to believe this.

First I tried a repair of Window 10, ran SFG /scannow and DISM both reported some problems weren't fixed.

BTW this is a new reset of my Windows 10 Pro system, performed on 2/23/16

I uninstalled SketchUp, then reinstalled it, as administrator. When I ran it, the first time, the Extension Store dialog popped up, no I did not install the extension, but its there. This stuff is way above my pay grade.

It's almost cocktail hour, so i'm done for the day. I'll think long and hard about a complete system restore to factory settings.

Everything was deleted, there was no ..._loader.rb file unless it was in the deleted folder.

I disabled all extensions and plugins, except the su_ ones, then I deleted the SketchUcation file and folder again (still no SketchUcation_loader.rb file, there is a SCF_loader.rb file in the subfolder), reinstalled it again through preferences and downloaded a fresh copy of SketchUpMake-en-x64.exe and did another repair. Ran SketchUp as administrator and got the dialog popup when SketchUp started!

I agree, there is more than likely an issue either with Windows 10 or my computer, funny though that only SketchUp seems to be affected.

This was first placed in the SketchUp forum:
"After resetting Windows 10 and reinstalling SketchUp and all my normal extensions, now when I open SUp, open a model, or click on File-New, the ExtensionStore from SketchUcation pops up, never happened before and I can't find a way to remove it without removing the SketchUcation extension.

Any ideas?"

Tig gave me several suggestions:
@unknownuser said:

When you say the ExtensionStore from SketchUcation pops up I assume you mean the dialog opens ?
The toolbar itself will be there if you have activated it.

I have the same set up as you and I don't see the issue [I assume you have SketchUp v2016].
Thousands of users do too, so something is up with your installation...

But... you are really posting in the wrong forum.
Please post in the appropriate thread over at SketchUcation...
Before that...
Can I suggest you open the SketchUp.../Plugins folder and find the SketchUcation .rb file and matching folder and manually delete them [if you have a file-named !SketchUcation_loader.rb then also delete that].
If you have copied files over manually you might have unexpectedly broken something - a clean installation, uses the latest RBZs is always recommended].
Then download the latest RBZ for the toolset and reinstall it using SketchUp's Preferences > Extensions > Install Extension... button.
To be doubly sure restart SketchUp...
Report over at SCF...

There is no way that the dialog's code gets activated without a user's deliberate button-click on its toolbar or the submenu item, or a shortcut-key is assigned and it is pressed [or perhaps you have done that and a keyboard glitch is happening?]

When you click on File > New does it open a New SKP, or just the dialog ?

Can I also suggest you reinstall / repair your SketchUp installation using the exe installer - making sure you right mouse-click and use 'run as administrator' NOT double-click it - even if you have admin-rights, unfortunately under Win10 SketchUp v2016 can install oddly, unless it is install by the real admin [i.e. please don't do it à la double-click as a 'user-with-admin-powers' - you are NOT the same thing].
After a reinstall/repair all preexisting plugins/extensions should be retained...

So after reinstalling the SCF toolset AND SketchUp...
Any improvement ?

Deleted the SketchUcation.rb file, it was the only thing listed that Tig said to delete.
As I reloaded a fresh copy of the RBZ file the dialog popped up!
Restarted SketchUp clicked on File-New and the dialog popped up!
Did a repair (as administrator) and restarted SketchUp, the dialog popped up!