The return that you are getting (what you posted in the [code] tags) is not good xml. It hangs up in the first line at related="..." It's the second equal sign in the url that is killing this. I'm not that familiar with Wolfram API, but the little I read said you can change what you get sent back by your request structure. You should look into that.
document.write(xmlDoc.getElementsByTagName("plaintext").childNodes.nodeValue + "<br>");