Hello, NeoDoc.

Thank you for contacting us.  Well, that is quite annoying.  I just uninstalled, reinstalled and tested our Chrome extension in both Chrome 46 (the current version) and Chrome 48 (a future version) and it did not give me any such crap, other than the Not from Chrome web store. And it works perfectly.

I understand that, a couple years ago, Google began requiring all extensions installed into Chrome running under Microsoft Windows to go through their store, but the last blog post I can find indicates that there are no plans to apply this restriction to OS X.

I wonder if maybe you've set Chrome into some kind of extra-safe mode, with parental controls on, or something like that?  In the upper left of your screenshot there is a popover with a hyperlink to "Learn More".  Please click that hyperlink in Chrome and tell us what you "learn".  If it opens a web page, send us the URL.