Go to:
https://console.developers.google.com/project/apps~YOURAPPID/apiui/credential
to the Public API access section. (Make sure you change the "YOURAPPID" section to your own.) If you don't already see the API key, click on the "CREATE NEW KEY" button.
Am not sure how this took you a week, but you can find information on API-KEYS for Google App Engine on the Google for developers website. With your Google account, you can set everything up in minutes in the API Console.