That's not the right page. Do this:
1. Click on any existing fact.
2. At the very bottom, click on the button "Add a new Source Citation"
3. You will then see a screen like the one I'm attaching below.
4. Click on the little down arrow or "Select a Source" (at the top) to see the list. You can scroll down through the list from the right side slider bar.
5. Or, to add a new source, click on the link below the button "Create a New Source"