Chrome not allowing me to install the extension

I dragged the extension to the extensions page as described in the README, but the enable button is dimmed out, and it says "Not from the Chrome Web Store". I don't see any way to enable it.

If there is some way to enable this extension (ideally without making my Chrome completely insecure), please update the README. Otherwise, why isn't this on the Chrome Web Store? I want to be a Cloud to Butt purist, but this makes it difficult.