When you're creating or editing a listing for an app, the Offering tab asks whether you want to offer a free trial of Salesforce and your application. Before you can offer the free trial, you must first request access by logging a case in your salesforce.com partner portal. After you are granted access, you will need to prepare a sample Salesforce organization that includes your app and sample data; this organization creates a trial template for customers who select the trial option. You must submit the trial template for security review and associate it with your app listing before you can make it public. Click here to learn more about free trials on the AppExchange.