To create a subscription/preapproval product you will want to go to your sellers dashboard and then click on create new product or you can edit an existing one. You will then need to scroll down to Currency and check the 'Subscription or Multiple Payment' box.
That will then open the subscription settings box for you. In that box, you will want to put how often the customer is charged, for how long, and what commission you want to payout on the first payment.
Under that is a section you can use to set up a trial period. You can see exactly where you need to set this up here: http://screencast.com/t/WSBgZrRE