Here's how you can do it. Please note however that it can take up to 36 hours for this information to post on our site.
Find the "detail page" for the listing (you can do that just by searching for the address on Trulia). In the top right corner of the detail page, you'll see a "Tools" section. In the Tools section, there's a link that says "Submit open house times". Click this link and fill in the relevant date and time info.
Let us know if you have any other questions.