If your story's chapters have titles beyond the default 'Chapter 1', 'Chapter 2', etc..., you can turn on TOC display in the story's cover/index page.
To do that:
-> Authors/Editors page
-> Click on the 'Manage Stories' button
-> Click the story's title
-> Scroll down to the end to see the chapter titles
-> Click the 'Show TOC' button.
Please note, that if your chapter have only the default titles, the 'Show TOC' button doesn't show up.