Basic Preloader by PineappleClock

Add a movieclip to your stage, then click on the movieclip and enter this actionscript:

onClipEvent(load) {
  //this part gets executed once, when the movieclip loads.
  stop(); //this stops the movie from playing.
}
 
onClipEvent(enterFrame) {
  if (_root.getBytesLoaded() == _root.getBytesTotal()) {
    //execute this part if the movie is loaded
    play();
  } 
}
 

This code is fine, but it starts the movie as soon as it loads. Usually you will want a button of some sort in order to allow the user to start the movie manually.

Preloader with precentage display

Create a movieclip, and a dynamic text object. Name the dynamic text object loadpercent and place it on the stage.

onClipEvent(load) {
  stop(); //this stops the movie from playing.
}
 
onClipEvent(enterFrame) {
  if (_root.getBytesLoaded() == _root.getBytesTotal()) {
    //execute this part if the movie is loaded
    play();
  } else {
    var loaded = _root.getBytesLoaded() / _root.getBytesTotal();
    _root.loadpercent.text = String(Math.round(loaded * 100)) + "%";
  }
}