Adding url to a button

[gmboat]

I am new to Action script. I am trying to add a url to a button but I get the following error: 1120 Access to undefined property btnLondon_btn

My code is as follows:

import flash.events.MouseEvent;
import flash.net.navigateToURL;
import flash.net.URLRequest;
import flash.net.URLRequestMethod;

btnLondon_btn.addEventListener (MouseEvent.CLICK, goLondon);
function goLondon (event:MouseEvent)
{
var url:String = "http://www.globillionaire.com/?page_id=806";
navigateToURL (new URLRequest(url));
}