Problem with AS3 codes

The compiler produces error code

"1120: Access of undefined property master_mc." (bolded)

Anyone know what went wrong?
I followed a tutorial but there wasn't any explanation on how master_mc really works.

Please help make this script run properly.

Here's the link to the tutorial I tried.

var xmlRequest:URLRequest = new URLRequest("imageData.xml");  
var xmlLoader:URLLoader = new URLLoader(xmlRequest);  
var imgData:XML;  
var imageLoader:Loader;  
var rawImage:String;  
var rawH:String;  
var rawW:String;  

var imgNum:Number = 0;  
var checkSec:Timer = new Timer(100);  
var numberOfChildren:Number;  

xmlLoader.addEventListener(Event.COMPLETE, xmlLoadedF);  
master_mc.addEventListener(MouseEvent.CLICK, nextImgF);//problem
master_mc.buttonMode = true;//problem  

function xmlLoadedF(event:Event):void{  
    checkSec.start();  
    checkSec.addEventListener(TimerEvent.TIMER, checkerF);  
    imgData = new XML(event.target.data);    
}

function packagedF():void{  
    checkSec.removeEventListener(TimerEvent.TIMER, checkerF);  
    rawImage = imgData.image[imgNum].imgURL;  
    numberOfChildren = imgData.*.length();  
    rawW = imgData.image[imgNum].imgW;  
    rawH = imgData.image[imgNum].imgH;  
    imageLoader = new Loader;  
    imageLoader.load(new URLRequest(rawImage));  
    master_mc.addChild(imageLoader);//problem  
    imageLoader.x = (stage.stageWidth - Number(rawW))/2;  
    imageLoader.y = (stage.stageHeight - Number(rawH))/2;  
}

function checkerF(event:TimerEvent):void{  
    if(imgNum == 0){  
        packagedF();  
    }  
    else if(imgNum < numberOfChildren){  
        imageLoader.unload();  
        packagedF();  
    }  
    else{  
        imageLoader.unload();  
        imgNum = 0;  
        packagedF();  
    }  
}  

function nextImgF(event:MouseEvent):void{  
    checkSec.addEventListener(TimerEvent.TIMER, checkerF);  
    imgNum++;  
}