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.

1. Select the site that contains your manual.
2. Select the manual and chapter to add it to.
3. Click Move article.