Thanks for your reply @cub . Your description has enlightened me. So the blue button is supposed to launch software centre! (the setup guide does not mention that)
The pop-up does not recognise the .flatpakref file & offers to save it or open as txt. (there are no other options available in the apps dropdown)
Flatpak support is enabled in Manjaro Software Manager as per the setup instructions (linked in first post) & I am using the latest version 21.2
Thanks for the terminal suggestion. Although that would be a workaround, its not really what this question is about, which is specifically why the blue button doesn’t work.
I can also search in the Software Centre & find the FlatPak apps & install them there, which would be perfectly acceptable. But means I would bypass the flathub website completely & still leaves unanswered why the blue button & setup guide doesnt work.