App Engine actually recently added the ability for the developer who uploaded a given app version to download its source code.
check if your app is uploaded with same email id that is in your app engine. if you are not sure then in app engine > control > Clear deployment credentials and then click on any project, deploy to sign in again then use this
appcfg.py download_app -A {app id from google app engine} -V {1} "{c:\path}" --oauth2_credential_file=C:\Users\{your account name}/.appcfg_oauth2_tokens
change all {} to your needs