First, go to http://cloud.google.com/console and log in. Select your project and then click the Cloud Storage link. You’ll see the screen below:
Check the checkboxes next to the Cloud Storage buckets you’re using, then press the button marked “Bucket Permissions”. You’ll see the following screen:
Add your App Engine application by setting the first drop down box to “User”, the textbox to “(your application id)@appspot.gserviceaccount.com”, and the second drop down box to “Owner”.