Thanks for posting in the Tableau Community. I am going to move this post into the Developer Forums, which is the best place to ask questions regarding the Tableau Extensions API.
The link appears to go to a non-working page. Extensions for Tableau dashboards can be found here: Save and Share Tableau Prep Work - Tableau Is the issue occurring with one of the extensions listed in the gallery?
Additionally, the extensions are created by various different authors all who also offer their own support for their extension.
Information from: Use Dashboard Extensions - Tableau
Hope this helps!