Click on the "New Message" button to create a new message for your client.
Choose the Client you wish to message by selecting them from the drop-down list.
Write a message to your client, by clicking the name in the drop-down list. Then insert a subject and a message, and click the orange "Send Message" button!
Additionally, you can view messages from your Clients by selecting their profile to the left in the message center.
Congratulations, you just messaged your client.