If you want to include Products on your Estimates, you must have a Service on the Estimate first. Adding Products for Estimates is largely the same as adding Products for use on Jobs.
If you want to show the Product on Estimates, click the Estimates tab on the Add or Edit Product overlay:
From there, you can add an Estimate Description. If one is not added here, the text on the Estimate will default to the Invoice description.
Don't forget you can also set up a rate matrix on any Product. This is useful if you price a Product based on a Custom Field, such as square footage. The Rate Matrix pricing will pull into an Estimate if the Product is used on the Estimate.