If you have purchased the Sales Page Builder Addon and have already created your store, then from within your JVZoo My Purchases portal click "View Details" next to the My Product Store Sales Page Builder product. Then  click the 'Access Your Purchase' button to activate the add on in your store.

The sales page builder is accessed via the add/edit products section. Once you add or edit a provide you will see the Sales Page Builder button where is can be accessed, as show in this image.