The Free Offers Add-on is not available for individual purchase. It is included in the Exchange Pro Pack, Developer Suite or Toolkit.
The Free Offers add-on allows you to create free products with custom Buy Now and Purchase button text.
Once you've installed and activated the Free Offers add-on, you'll see Free Offers in the Advanced Options of your products.
In the options for Free Offers, you can chose to hid the product price on the Exchange store and product pages and also customize your Buy Now and Purchase button text.