The Extensions Gallery is in Beta because we are just starting out with it and still working on a few kinks. However, Extensions is not in Beta anymore! To use them you need the latest release of Tableau, 2018.2. You can download this from here: Tableau Desktop 2018.2 | Tableau Software and you can check this page on how to use them: Use Dashboard Extensions. Just download any extension from the Gallery and you're good to go!