Minor thing done, but plugin still not installing…
I simply managed to get the Manage Plugins options to appear in Mail.app using this command then entering my password in Terminal [thanks to last thread post from user Dialabrain here].
sudo defaults write "/Library/Preferences/com.apple.mail" EnableBundles 1
But per second pic, none show as installed, due to the DT install error message (shown in the first pic in the first post).