The new update to Google Merchant Center changed everything around. If you're like me and like to use a spreadsheet where possible, using supplemental feeds for product info testing is handy. But in the new update, it's nowhere to be found. After a bit of digging, and no help from Google documentation I found it. Here's how to activate the supplemental feature. 1. Go to settings in your GMC and then to add-ons. 2. Find "Advanced data source management" and add it. 3. Go to "Data Sources" 4. Under data sources "Provided By You" you should see "Supplemental Sources". If it's not there right away do a refresh. Depending on your changes it might take a while to see any data related to the changes after you add your new supplemental feed. But I am curious if others who had feeds set up already - was this already active for you? It wasn't for me.