The samples provided with that code are not what it says on the tin. They are not samples, they are scaffolds to play with - meaning: without changes they don't do a thing. In your case, look at lines 70+ in Samples/CreateFulfillmentOrderSample.php
:
// @TODO: set request. Action can be passed as FBAOutboundServiceMWS_Model_CreateFulfillmentOrderRequest
$request = new FBAOutboundServiceMWS_Model_CreateFulfillmentOrderRequest();
// $request->setSellerId(SELLER_ID);
// invokeCreateFulfillmentOrder($service, $request);
If you run it, you should get an error message "Unable to construct from provided data. Please be sure to pass associative array or DOMElement".
In your case, the XML should be passed as a DOMElement class parameter, so change above code to pass that parameter:
$request = new FBAOutboundServiceMWS_Model_CreateFulfillmentOrderRequest($DOMElement);