Home > Setup (Content) > Manage Products > Related Products
In v10, you will need to activate related products by creating a new setting called RelatedProducts.NumberDisplayed:
Give the 'default store' (and any/all stores that you are running) values for the number of related products that should show up.
The related products feature gives store admins the ability to cross-sell or recommend additional products to their customers. There are 2 ways to set up related products: Pre-Determined, and Dynamic.