To add a product to the lead click on the lead in the pipeline.
Click on Products from the leads details page and enter the name of the product which you want to add to the lead. If its an existing product i.e. created previously either using the Products option or through another lead then the product name will appear in the auto-complete. Selecting the product name will fill in the product price automatically.
Enter the product quantity and click on Add.
The value of the lead will be automatically set as the cumulative value of the products added to the lead.