Modal notification box on the screen of every authenticated user I want that every time I or any other admin (I have 2 more) post a new article each authenticated user should get a modal notification box on it's screen displaying a teaser of the Content Type I have posted. 

I created an action 'Display a message to the user' for the trigger, 'TRIGGER: AFTER SAVING NEW CONTENT' and then tried saving new content and checked on a different account (which was already logged) for any message but nothing happened. Also, if this worked I'd like to show a modal box with the display settings (which I have in place already).
Drupal does not provide a way to send notifications to all site users by default. The action you are trying to use will only notify the user who triggers the action (you). Messaging can help you achieve what you want. You may also need the appbar module.