Go to "My Listings" - either the pull down menu or click on "Listings." If you have not identified the address as your listing, you want to do that. Then look for "open house" when you fill out all the information for your property.
Yes! The house will need to be listed on Trulia. Trulia is a syndication site. Feeds are not directly from the MLS typically from Broker sites so it may take a couple of days for the listing to get to Trulia.
Once the listing is on the site claim the listing which is easy to do through your profile page. After it is claimed you can click on the property and indicate an open house and put the date and the time.