There are 2 previous versions of this script.
// ==UserScript==
// @name MegaUpload Auto Start Download
// @namespace Laymain
// @description MegaUpload Auto Start Download
// @include http://*.megaupload.com/?d=*
// ==/UserScript==
setTimeout('if ((dlLink = document.getElementById("downloadlink")))window.location.href = dlLink.firstChild.href;',
parseInt(document.getElementById("countdown").innerHTML)*1000+1);