According to the plug-in's description: "You haven't saved your free Wapple Architect Dev Key, get one from here: http://wapple.net/signup/wordpress". There is no such item on that page anymore. The site requires you to buy a 5 pound per month or more subscription.
I understand this is business and people have a right to make money with the work they do. However, the statements made within the plug-in are incorrect. Either they should be correct, or you need to remove that statement from the plug-in you release to WordPress.