The Official answer is you need to have an Agent put it on the site for you. If you have an Agent you can have them go here and list it on Trulia for you (It's Free)..
If you are doing an FSBO then you can find any of the many Flat-Fee services to list your Home here and on other sites like Realtor.com, MLS ect...
Also you can choose to follow Trulias response to this question and contact them...
From Emily of Trulia....."If you are selling your home on your own, please note that Trulia does not include FSBO listings within our search results, however you can talk to our sales team to learn more about advertising options that you may may be able to take advantage of. Please call 888-568-6487 if you'd like to learn more. "
Good luck, Dunes