You need an active Premium Account to reply to a listing and message the landlord.
You can choose from a 4-week or 12-week subscription, or a one-time 6-month plan.
After you have set up your Premium account, you can head to the listing you want to reply to and write a message in the text box at the bottom of the page.
When you've hit the 'Send message' button, it goes to the landlord.