Thanks for your question here. The email sent out to which I believe you refer offers a one month rebate code. What the system requires if for you to enter your billing information into the Trulia system. Upon this action you will be charged $39 for your first month of Trulia Pro service.
However, with entry of your rebate code, within 24 hours of signing up you will see that your credit card has a credit back to it for the same $39. Net, you should have a free month of Trulia Pro service.
If the product works for you, you can keep the service and Trulia will bill you the next month for the service. If you do not like the service and it does not work, you can cancel at any point.
Hope this helps and I hope you give Trulia Pro a shot. Lots of people have been having success with Trulia Pro, check out what some of our early users think below.