Once you are logged in, go to the listing and then to "Edit listing info" and scroll down to the Open House section. You can also click on "Edit" and then "Add/edit open house dates."
You can add multiple dates. Once a date has passed, the next open house date will automatically appear. When finished, scroll to the bottom of the page and click on "Post for sale."
If you need to remove the open house, simply click on Add/Edit open house dates again from the "Edit" menu and select "remove."