ShortTag shows you how you can create a Button with AS3:

var sb:SimpleButton = new SimpleButton();
//we create the SimpleButton object

//as you know a Button in Flash has 4 state: up, down, over and hit
//we create a MovieClip for the up, down and hit state and an other one
//for the over state

var mc:MovieClip = new MovieClip();
//we create theĀ  MovieClip object×00FF0000);,0,200,100);

var mc2:MovieClip = new MovieClip();×0000FF00);,0,200,100);

//We have drawn a 200×100 pt button, upState is colored with red, on Over is colored with green

//Now we assign the MovieClips to the Button

sb.upState = mc;
sb.overState = mc2;
sb.downState = mc;
sb.hitTestState = mc;

//Be aware to assign all for states otherwise your button will appear but will not act like a button

//And finally we add the MovieClip to the current Container