AS3 How to add adMob to Adobe Flash/Animate?

Please help me to figure out how to add adMMob to my Actionscript 3 app. There isn't well-written documentation on how to do it. I only found a youtube video that explains it but still, it was not well and fully described. for the bits and pieces of information that I gathered, I managed to construct the following code, however, it errors. Please help to reconstruct the code so I get it to work.

1. First I got Pozirk's AdMob Air Native Extension from github
2. I installed the AdMob.swc and the AdMob.ane files in the Advanced ActionScript settings
3. I created a class file called Main.as which contains the following code:

package 
{	import flash.display.MovieClip;
	import com.pozirk.ads.admob.AdMob;
	import com.pozirk.ads.admob.AdParams;
	import com.pozirk.ads.admob.AdEvent;
	var _admob: AdMob = new AdMob();
	public class Main extends MovieClip
	{
		public function Main()
		{
			//> initialization of AdMob
			_admob.addEventListener(AdEvent.INIT_OK, onEvent);
			_admob.addEventListener(AdEvent.INIT_FAIL, onEvent);
			_admob.addEventListener(AdEvent.BANNER_SHOW_OK, onEvent);
			_admob.addEventListener(AdEvent.BANNER_SHOW_FAIL, onEvent);
			_admob.addEventListener(AdEvent.BANNER_LEFT_APP, onEvent);
			_admob.addEventListener(AdEvent.BANNER_OPENED, onEvent);
			_admob.addEventListener(AdEvent.BANNER_CLOSED, onEvent);
			_admob.addEventListener(AdEvent.INTERSTITIAL_SHOW_OK, onEvent);
			_admob.addEventListener(AdEvent.INTERSTITIAL_SHOW_FAIL, onEvent);
			_admob.addEventListener(AdEvent.INTERSTITIAL_CACHE_OK, onEvent);
			_admob.addEventListener(AdEvent.INTERSTITIAL_CACHE_FAIL, onEvent);
			_admob.addEventListener(AdEvent.INTERSTITIAL_LEFT_APP, onEvent);
			_admob.addEventListener(AdEvent.INTERSTITIAL_OPENED, onEvent);
			_admob.addEventListener(AdEvent.INTERSTITIAL_CLOSED, onEvent);
			_admob.addEventListener(AdEvent.REWARDED_CACHE_FAIL, onEvent);
			_admob.addEventListener(AdEvent.REWARDED_CACHE_OK, onEvent);
			_admob.addEventListener(AdEvent.REWARDED_CLOSED, onEvent);
			_admob.addEventListener(AdEvent.REWARDED_COMPLETED, onEvent);
			_admob.addEventListener(AdEvent.REWARDED_LEFT_APP, onEvent);
			_admob.addEventListener(AdEvent.REWARDED_OPENED, onEvent);
			_admob.addEventListener(AdEvent.REWARDED_REWARDED, onEvent);
			_admob.addEventListener(AdEvent.REWARDED_STARTED, onEvent);
			_admob.init();
		}
	}

}

Then I added this line to the script, it's sitting on the top root and not part of a function or anything else. I'm not sure if it's supposed to be nested somewhere.

_admob.show("ca-app-pub-3940256099942544/6300978111", AdParams.SIZE_SMART_BANNER, AdParams.HALIGN_CENTER, AdParams.VALIGN_BOTTOM);

with this above settings when trying to compile the code I get the following error message:

"Main.as, Line 1 5006: An ActionScript file can not have more than one externally visible definition: _admob, Main"

 

1. Why I'm getting this error message?
2. Is this code is proper to run adMob?
3. am I missing something?