To add an open house to your listing, sign into your Zillow profile and navigate to your listing’s property page. Please make sure you are on the 'Public' view of the listing.
Then, select the More dropdown in the blue menu bar at the top of the page, and click "Add/edit open house dates."
Here you can input your open house details in the appropriate field.
To save your changes, select "Update for sale by owner" at the bottom of the page. You can then return to your listing to see your changes.