[andr64]

EnterFrame нада удалять -)

this.createEmptyMovieClip("mc", 0);
mc.lineStyle(0, 0);
mc.beginFill(0);
mc.lineTo(100, 0);
mc.lineTo(100, 100);
mc.lineTo(0, 100);
mc.lineTo(0, 0);
mc.endFill();
mc._alpha = 0;
this.onEnterFrame = function() {
if (mc._alpha<100) {
mc._alpha += 3;
} else {
delete this.onEnterFrame;
}
trace(mc._alpha);
};