The Official answer is you need to have an Agent put it on the site for you. If you have an Agent they can 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 on the MLS, Trulia and on other sites like realtor.com, msn.com, remax.com, kw.com, prudential.com, realestate.com, excite.com ...
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 to learn more."
My suggestion to a FSBO would be to USE the Flat-fee and get on your MLS, Realtor.com. ect....That's far more important and valuable to you than listing on a site like Trulia and that's just the bottom line truth.
Listing on Trulia MIGHT give you some local exposure to buyers but your local MLS is what you want your home listed/displayed on for your Market area and so all Agents are aware of it, know what commission you offer plus buyers may ask their Agents about it if they see it on the MLS.
Then Realtor. com and on the other major RE company sites..Century, Re- Max, Prudential...then consider the semi- accurate listing messes like Trulia and Zillow.....Keep the priorities straight, you want to sell.
Flat-fee will get you on those sites (and Trulia/Zillow) cheaply and easily.
Another Suggestion..Ignore SPAM
How you decide to sell your home is your decision and every person has options to chose from depending on their unique situation or knowledge of the process.
Here's some information from the Dept of Justice on Commission %'s, Rebates, Consumer rights, filing complaints and much more....http://www.justice.gov/atr/public/real_estate/index.htm
Do your research for local Realtors that have the best independent internet advertising because that's how home buyers are searching for their new dream home. Once you have found the Agent that has their own website and has the best creative internet marketing strategy from any other Agent and Real Estate Company that's the agent you should be interviewing. Placing a real estate sign on your lawn and uploading to the your local MLS is just not enough in today's market.
Best of luck in hiring your next Realtor with the best internet marketing and your property should appear in Trulia.