Wow! I had no idea 1.1.0 had been released. For some reason the private NuGet feed I setup as per https://docs.telerik.com/blazor-ui/installation/nuget is still showing 0.5.0 as the only available version.
I downloaded the .zip archive from my "Downloads" page and the "Add" button now works. So thank you for that!
However it would be great to hear any tips on why my private nuget feed is still just showing 0.5.0...