Something like this would do:
var atimer:Timer = new Timer(500,30);
atimer.addEventListener(TimerEvent.TIMER,addone);
atimer.start();
function addone(e:TimerEvent){
var anmc:MyMovieClip = new MyMovieClip();
addChild(anmc);
}
As that stands the movieclips ("MyMovieClip" is the name you
used when you set that symbol for sharing in the Library) would be
added every half second, but they would all be on top of each
other. You probably would want to set the anmc.x and anmc.y to the
position you need before doing the addChild().
The event has a property called currentCount, which you could
use to help figure out where this one gets placed. Like:
anmc.x = e.currentCount * 20;
would spread the 30 movieclips across the stage, 20 pixels
further along each time.