Gain access to directory

Hey guys, I am trying to write a code that downloads "extra" content to the users computer after the application has been downloaded (to reduce download time and save space). So far I have only researched it, and attempted to try with a copy-and-pasted bit of code. I am getting an "UnauthorizedAccessException", so I asume that I can't download a file from the internet without the users permission; and thats why I am writing this question.

My code (mainly from http://www.csharp-examples.net/download-files) as well as the error I am currently getting are at the bottom, respectively.

 using System;
 using System.IO;
 using System.Net;
 using UnityEngine;
 using System.Security.AccessControl;
 //Written by Gibson Bethke
 public class Download : MonoBehaviour {
     
     static public string newFile = "/Users/" + Environment.UserName + "/Desktop/Downloads";
 
     void Start () {
         print(newFile);
         if (Directory.Exists(newFile)) {
         } else {
             Directory.CreateDirectory (newFile);
         }
     }
     
     void OnGUI () {
         if (GUI.Button(new Rect(1300,875,100,50), "Download")) {
             WebClient webClient = new WebClient();
             webClient.DownloadFile("http://db.tt/qXWBNu3g", newFile);
         }
     }
 }

 UnauthorizedAccessException: Access to the path '/Users/************/Desktop/Downloads' is denied.
 System.IO.FileStream..ctor (System.String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, Boolean anonymous, FileOptions options)
 System.IO.FileStream..ctor (System.String path, FileMode mode)
 (wrapper remoting-invoke-with-check) System.IO.FileStream:.ctor (string,System.IO.FileMode)
 System.Net.WebClient.DownloadFileCore (System.Uri address, System.String fileName, System.Object userToken)
 System.Net.WebClient.DownloadFile (System.Uri address, System.String fileName)
 Rethrow as WebException: An error occurred performing a WebClient request.
 System.Net.WebClient.DownloadFile (System.Uri address, System.String fileName)
 System.Net.WebClient.DownloadFile (System.String address, System.String fileName)
 (wrapper remoting-invoke-with-check) System.Net.WebClient:DownloadFile (string,string)
 Download.OnGUI () (at Assets/Download From Web/Download.cs:22)

So, if anyone has any advice, I will really appreciate if you share it with us/me.

Thanks for all your help! Gibson

P.S. I am aware that the file I am currently trying to download is not "extra" content, but a hand-drawn image of a pony. P.P.S. I am not trying to do this in/with a web player; I am trying it in the engine itself, and I intend to use it in a standalone.