Can't be done!
Unless I've misunderstood what you're trying to do, but even if I am, you are trying to access cookies on one domain to another, which isn't possible. Cookies are generally domain specific.
I assume you're trying to load cookies from a server side script. The IFRAME is only executed on a page load, and a page only begins to load once the server side script starts outputting markup to the browser. The contents of the IFRAME are not available to your script, as the loading of the IFRAME is a client side process.