Hi, You have to use a site that syndicates your listings to Trulia or you can manually post. I personally use point2agent and they syndicate all of my listings to Trulia as well as many other sites on the internet. There are other similar systems that you can use if you don't have a website that will do it for you.

Trulia needs either a feed from your website or your comapnies site. Trulia does not extract listings from MLS's as that costly.
If you have a website with your own listings on them then you can submit that site to be crawled by trulia or you can set up a feed.
Check out the trulia help section to walk you through it.