I would like to ask which kind of credentials do I need to put on for importing data using the Google App Engine BulkLoader class
appcfg.py upload_data --config_
I recommend you follow the advice given here, and I quote:
add this to app.yaml file:
-- url: /loadusers script: myloader.py login: admin
Note that if you run it on local development machine, comment off the last line login:admin so that you don't need a credential to run the bulkloader.
(my emphasis).