There are not many people who do it but still sometimes we come across third party browser extensions for e.g Yesterday I was installing a download manager called "FlashGet" (Try it it's pretty good). It works with two of the leading browsers that is "Mozilla Firefox" and "Google Chrome". The extension for this very software was directly available in the Firefox add-on page but it wasn't there on the "Chrome web store". Instead, when I chose to install the browser support for Chrome it just downloaded a ".crx" file and I got this error. So it came to my knowledge that the new Google Chrome can't apply off-store plugins.
So, to fix this, you can try 2 things:
1. Download the extension-->Open the extension page from the menu-->Drag the downloaded extension to the page
2. Or there's a permanent solution: Right click on Chrome icon-->Properties-->add "--enable-easy-off-store-extension-install" without quotes to the target at the end and apply. Now you can install any off-store plugin!