You might want to use joFileSource instead; it's a higher level data source class. An example can be found in jo/samples/file/file.js (also this working demo).
You may also be running into cross-site security issues with your URL if it's hosted somewhere other than where your index.html was served, or some mobile OS' require your app to have special permission to do so. In general, I prefer jsonp to xmlhttprequest for that sort of reason.