If you're trying to install the extension in Safari, but it's not appearing in your Safari preferences (Safari > Settings > Extensions), please try to following:
- Open Terminal on your Mac.
- Paste the following command into it and hit Enter:
/System/Library/Frameworks/CoreServices.framework/Frameworks/LaunchServices.framework/Support/lsregister -f -R /Applications/Safari.app - Open Safari's preferences again - you should see the extension listed.