You will need to log in to your google merchant center, then create a new feed. To do so go to Data Feeds, and click on "New Data Feed". Leave the settings as they are and put in the filename, that can be anything as long as it ends with ".xml" or ".txt" depending on the type of feed you use.
Once you save it will take you back to the feed list, find the one you created and click on "Create schedule". Set it for anything you like and use the URL found in your admin panel.
