You need to dynamically attach a MovieClip from the library.
Use the MovieClip constructor to attach a symbol from the library.
attachMovie("my_clip", "clip", 10);
clip._x = 100;
clip._y = 100;
clip._alpha = 50;
var clip:MovieClip = new my_clip(); clip.x = 100; clip.y = 100; clip.alpha = .5; addChild(clip);
Note: In ActionScript 2 the linkage ID, "my_clip", is set in the library for the MovieClip, whereas in ActionScript 3, the class name, "my_clip", is set.