How to add alert in the flash page?

1

Hello, friends. I wonder how to add the alert into the flash page. I have situation like using buttons to control the movement of the movieclip. I would like that user first click on the movieclip which he want to move before he press the button for moving up and down. Then if the user press the button without clicking the movie clip, there will be the alert message to say that “You have to click on the movieclip first”. Could anyone give some hint how to do it ?Thanks very much in advance! Regards!