Looks like Sander just did an update to the Railcasts channel to bring it up to the most recent framework specs. You can try again to add your DefaultPrefs.json and you'll need to add a line to the __init__.py file to add a Preferences object to the menu like this one in the Pandora channel:
oc.add(PrefsObject(title='Change your Pandora Preferences', thumb=R(ICON_PREFS)))