This test will only work with a mobile user agent. It will not work if you open it in a desktop browser with a standard user agent! To run this test you should use this Firefox extension: https://addons.mozilla.org/en-US/firefox/addon/user-agent-switcher/ and switch the user agent to be, eg, iPhone 3.0. This is because the ads are targeted to mobile devices only.
The ad should appear horizontally centered in the browser window.
320x50 IU showing an xFP ad
6:1 Aspect ratio IU showing xFP ad