To communicate with your guests via search and stay, you will firstly need to log in to your account.  

From the dashboard, select the 'Bookings' tab in the left hand tool bar.  Select the relevant booking  by clicking 'View' on the right hand side in the 'Action' column.  This will pull up the customer details and payment information.  Below the customer details you can click on the 'View Messages' button,  scroll down you will see a text box under a heading 'Send Message' type your message here and click the  'Send' button below.

These messages can be viewed under the 'Messages' tab in the left hand toolbar.