Traditional Cross-Sells and Up-Sells are accessed via the Related tab whenever you edit a Product, Lesson or Course.
To add a new Related product, simply hit the New Cross Sell button.
Select the product you want to link. NOTE: You can select more than one product at a time.
If you want this to be an Up-Sell only, check the Upsell Only box. Otherwise the related product will also link back to the product you're editing.