public class EllipticMotion extends MovieClip {
  // ...[中略]...
  public function EllipticMotion(nDegree:Number, myCenter:Point, myRadius:Point) {
  degree = nDegree;
  center = myCenter;
  radius = myRadius;
  addEventListener(Event.ENTER_FRAME, rotate);   // 後に削除
  setRotation();
}

var instances_array:Array = new Array();   // 追加: 配列を変数として宣言・初期化
for (var i:int = 0; i < nCount; i++) {
  var _mc:EllipticMotion = new EllipticMotion(nDegree * i,myCenter,myRadius);
  addChild(_mc);
  instances_array.push(_mc);   // 追加: 配列にインスタンスを加える
}

var deceleration:Number = 0.1;
// ...[中略]...
addEventListener(Event.ENTER_FRAME, rotate);
function rotate(EventObject:Event):void {   // 追加: リスナー関数
  var nSpeed:Number = (stage.mouseX - myCenter.x)*deceleration;
  var nLength:uint = instances_array.length;
  for (var i:int = 0; i<nLength; i++) {
    var _mc:EllipticMotion = instances_array[i];   // インスタンスの取出し
    _mc.rotate(nSpeed);   // インスタンスのrotate()メソッドを呼出す
  }
}

// タイムライン: メイン
var myCenter:Point = new Point(stage.stageWidth / 2, stage.stageHeight / 2);
var myRadius:Point = new Point(100, 50);
var nCount:uint = 6;
var nDegree:Number = 360 / nCount;
var instances_array:Array = new Array();
var deceleration:Number = 0.1;
for (var i:int = 0; i < nCount; i++) {
  var _mc:EllipticMotion = new EllipticMotion(nDegree * i, myCenter, myRadius);
  addChild(_mc);
  instances_array.push(_mc);
}
addEventListener(Event.ENTER_FRAME, rotate);
function rotate(EventObject:Event):void {
  var nSpeed:Number = (stage.mouseX - myCenter.x) * deceleration;
  var nLength:uint = instances_array.length;
  for (var i:int = 0; i < nLength; i++) {
    var _mc:EllipticMotion = instances_array[i];
    _mc.rotate(nSpeed);
  }
}