I'm not sure what you have done wrong?
I made a project with standard flash export settings and it worked directly?
I've tried it locally and on a live server.
Here is the code:
import flash.display.MovieClip;import flash.events.MouseEvent;
var buttonA:MovieClip = new MovieClip();
buttonA.x = stage.stageWidth/2-buttonA.width/2;
buttonA.y = stage.stageHeight/2-buttonA.height/2;