Posting a listing on Trulia is absolutely free! We do not charge for listing inclusion.
Please do keep in mind Trulia only shows listings that are currently represented by an agent or broker. If you have an agent, all you need to do is tell them about us and point them to the page I'm linking to below. They can fill out a simple form to tell us about themselves.
Thanks for your interest!
Follow this link, it will guide you to submit listings via a web crawl or an XML feed. If you don't have access to sumbit this way directly, or through your broker, you can also upload your lisitngs to Point2Agent using a form and they can upload to Trulia and other sites also.
Here is the link for direct submission to Trulia