I figured out why, there were two problems, one my fault and one apparently Scirra's fault.
in the config.xml root element <\widget> that contains the xml name space i have not seen the id attribute that needs to match the application package.
when you export from Construct2 for PhoneGap Build you get a standard index.html page that targets to an ipotetical "phonegap.js" that surely is added by phonegap build but if your creating projects locally you get "cordova-2.X.X.js" and this breaks the game.
Answering the question for other readers purpose Phonegap and Phonegap Build work exactly the same way, just complain the standards!!