You may want to filter the articles. For example, if you are looking for articles that are orphaned (not associated with a manual) then you would check the Orphaned articles only checkbox.
Find the article you want to add and click the dropdown icon. If the article is already associated with a manual then you will see a Move option (1) in the menu. If the article is not associated with a manual then you will see the Add to Chapter option (2) in the menu.