Download assetBundle from server

Hi every one, im trying to donwload asset bundle and then instantiate any gameobject from that. But i dont understand what im doing wrong. i was watching some tutorials, but still doeanst work. Canu help me pls?
When im strarting Scene console write me this error :

NullReferenceException: Object reference not set to an instance of an object
loadAsset+c__Iterator0.MoveNext () (at Assets/Scripts/loadAsset.cs:16)

and here is my script :

using UnityEngine;
using System.Collections;

public class loadAsset : MonoBehaviour {

	private string url = "";

	IEnumerator Start () {
		WWW www = WWW.LoadFromCacheOrDownload (url,1);

		yield return www;

		AssetBundle bundle = www.assetBundle;

		GameObject kocka = bundle.Load("Cube", typeof(GameObject)) as GameObject;


		kocka.transform.Translate (new Vector3(1,1,1));


i was trying to use other server but still doeasnt work.
Can u help with my problem?
oh sure script its name is “ExportAssetBundles” i have in Editor Folder in my project.

Thanks Thomas

The link is broken: Dropbox - Error - Simplify your life

When working with WWW always check that WWW.error is null after “yield return www;”.

U can try uploading your unity3d files to either google sites or maybe use WinSCP as a server.