In this way I have something that I could call everywhere without instantiating it around the code.
For example I create a singleton class called Popup, that has a show method Popup.show(this, "my message"); where "this" is the reference of the display object I want to add/remove the popup.
However it is my reasoning, I really never tested it in a real project.
Create a movieclip and control its visible property via whatever you intend to use to make it appear. If you need the message to be dynamic, use a dynamic textfield.