Note: The Products feature is available in the Gold plan. To switch to the Gold plan, go to Settings > Billing > Change plans.
In the Detail View of the Deals tab, click on 'Add products', found beneath the Deal Title.
When clicked, you will be prompted to search for the name of the product you wish to add to the deal.
If no product appears, you will be asked if you wish to add that product to your company Pipedrive account.