To add Upsells/Related products to a product:

There are three types of product relations in Magento: Up-sells, Related Products, and Cross-sell Products.

Upsells 
When viewing a product, Upsells for this product are items that your customers would ideally buy instead of the product they’re viewing. They might be better quality, produce a higher profit margin, be more popular, etc. These appear on the product info page.

Related products
Related products appear in the product info page as well, in the right column. Related products are meant to be purchased in addition to the item the customer is viewing.

Cross-sells
Finally, Cross-sell items appear in the shopping cart. When a customer navigates to the shopping cart (this can happen automatically after adding a product, or not), the cross-sells block displays a selection of items marked as cross-sells to the items already in the cart.

To add Upsells/Related products to a product:

1. Log in to the Magento admin
2. Navigate to Catalog -> Manage Products
3. Find the product to which you want to add Upsells/Related products
4. Select and edit this product
5. Select the Upsells/Related products tab