NPAPI plugins have been around since Netscape 2.0(!) and extend the capabilities of web browsers, but they were designed in a less security-conscious time. These plugins are not sandboxed and have the same access to your computer as your current user account.1 This kind of access is prohibited in the new Windows Store apps, and so these plugins do not run in web browsers running in Windows 8's new UI. As such, Google has decided to block their installation under Windows 8.
When you view an extension that relies on one of these plugins in the Chrome Web Store, you will see the message: "This application is not supported on this computer. Installation has been disabled." However, these plugins still work in Chrome while running in desktop mode! Until developers update their software to use newer API's, you still may want to install these plugins for use on the desktop. Here's how.
If you don't mind restarting your browser, you can change the compatibility mode to Windows 7. Just right click on a shortcut to Chrome, and go to
Properties. Click the
Compatibility tab and select the
Run this program in compatibility mode for: checkbox. From the dropdown list select
Windows 7 and then click
OK. Run Chrome, and you should be able to install the extension. Once you're finished, you can reset your compatibility settings, and the extension will still work.
If you do not wish to restart your browser, you simply need to download the extension bundle and install it manually. This blog post has a solution. However, I find it easier to just use the website http://chrome-extension-downloader.com/. Copy and paste the URL from the Chrome Web Store into the search box, and the extension bundle will download to your computer. Then go to your Chrome menu, select
Tools and then
Extensions. Drag the bundle from your downloads folder onto the extension list. Chrome will ask you if you want to install the extension, and it will install without issue. Now you can use your NPAPI plugin in desktop mode without issue.