wow - Google did yet another change. Anyways.

After enabled GCM for Android, go to "Credentials". On the "Public API access" area, click the red "create new key" button.

Afterwards a popup comes; Here use the server key:
http://people.apache.org/~matzew/GCM_console.png

That's the same server key as we discuss in our tutorial.

I will update the docs ASAP, but in the meantime I hope this helped you to continue.

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