You add books to our sales on the pricing page for each book.
For example, if you go here in the breadcrumb menu at the top of the page on Leanpub:
Author > Books > [Book Title] > Store > Pricing
...you will see a box that says "MAXIMUM LEANPUB DISCOUNT %". Enter a number here, and click the blue "Update Pricing" button. Then, your book will be eligible to be included in a Leanpub sale.