Hi,
Sorry @smudger,
It is address in this thread: SOLUTION: "An error occurred when attempting to load this plug-in"
After running the installer, go to the plugin manager in Lightroom (file > plugin manager). Click “Add” to add a plugin, and then navigate to “home > plugins” — there should be a file in there named “NegativeLabPro.lrplugin” - that is what you want to add.
There was an issue when Apple updated their OS that made part of the installer not work (the part that tells Lightroom which folder to default to when you open the Plugin Manager). So for the time being, you just need to navigate to that folder (“home > plugins”). That is the same thing as “users/YourUserName/Plugins”