How to activate a Chrome extension not available on Chrome Web Store?

You should be able to unpack the crx to a directory by opening it with an unzip program, for example 7-Zip, Windows Explorer, or WinZip. If you are having a hard time opening it in your zip program of choice, just rename the .crx to a .zip and try again.

Extract the contents to a directory. Then in Chrome, under Settings, go to Extensions, then click "Load unpacked extnesion..." and pick your directory. That should install it. If you get an error, post exactly what error you get.

Download the extension, go to chrome://extensions, and drag the downloaded file onto the window.