You can sign up for push messaging with the demo as-is. However, if you want to test it and actually send a real push message you will need to provide your own details. Open up the manifest.json file and change the gcm_sender_id to your own, and then re-enable messaging to get a new subscription ID.
See the HTML5Rocks guide to push notifications for details on signing up for the necessary key and sender ID.

Enable push messaging
Current subscription

            
Send a push (Mac/Unix)
curl https://android.googleapis.com/gcm/send -d "registration_id=" --header "Authorization: key=[YOUR_PUBLIC_API_KEY]"