I can not find a working example of the new amazon service (or at least, within the last couple of years). The closest working example just comes back with a null item no ma
I believe your problem may be lack of an Associate Tag. As of November, 2011, this is required for all requests and I noticed early on in my testing that I got null responses back (with an error code) when I didn't include it. I'm not sure if that's still the behavior but I'd definitely assume that if you aren't adding it (which I don't see in your code) that's a likely suspect.
Look at top change note here
If you don't have an Associate ID you will need to apply for one.