Thanks Johnny2528,

I'v made some minor changes to your script and now it works.

PHP Code:
import flash.display.Bitmap;
import flash.display.BitmapData;
import flash.utils.ByteArray;
import flash.geom.Rectangle;
import flash.net.SharedObject;
import flash.events.*;
import flash.display.DisplayObject;
import flash.display.Graphics;
import flash.display.Shape;
import flash.display.Sprite;

createEmptyMovieClip("drawingPad",1);
drawingPad.createEmptyMovieClip("drawing",1);

var 
drawingFlag:Boolean false;
drawingPad.onMouseDown = function() {
    
drawingPad.drawing.lineStyle(5,0xFF0000,100);
    
drawingFlag true;
    
drawingPad.drawing.moveTo(this._xmouse,this._ymouse);
};
drawingPad.onMouseMove = function() {
    if (
drawingFlag) {
        
drawingPad.drawing.lineTo(this._xmouse,this._ymouse);
    }
};
drawingPad.onMouseUp = function() {
    
drawingFlag false;
};

function 
createBitmap() {
    var 
bitmapData:BitmapData = new BitmapData(drawingPad._widthdrawingPad._heighttrue0xFFFFFFFF);
    
bitmapData.draw(drawingPad);