Unable to get the Sample App "Push Notifications Sample" working

After cloning (today) the repository https://github.com/Icenium/sample-telerik-baas-push-notification and following the instructions, everything looks good in the Backend / Push Notifications screen - My Phone shows up in the list.

But my phone (Android 5.0) never gets my push notifications and the Backend Push Browser says 0% under "Sent To".

The App instructions (README.md) do not address the need for a Google Server Key.  The Backend area says that I need to add one, but then links to a Google page about Cloud Messaging - not Server Key.

In my Google Developers Console, I found "Credentials" under "APIs & apps".  I created an API key and put that in the Push Notifications / Settings.

But I still do not get a notification.

Thanks,

Ryp