First of all, use "Microsoft.XMLHTTP" instead of "Msxml2.XMLHTTP.3.0". It's strongly recommended that you try not to use version numbers in ActiveXObject references, and with "Microsoft.XMLHTTP" you're guaranteed that it'll always use the latest version.
Secondly, where did you define APIKey?
Apart from these remarks, I don't see the problem...
EDIT: Blah, Ezra beat me.