Userscripts.org

Exactly what you need to do, except change the id you're looking for to:

gamePage

so:

// ==UserScript==
// @name           Addicting Games - Skip Ad Before Game
// @namespace      RunningBlind
// @description    Skips the ad played before a game loads on AddictingGames.
// @include        http://www.addictinggames.com/*
// ==/UserScript==
if (document.getElementById("gamePage"))
	unsafeWindow.embedGameInDiv();