Create a new message

From your 'My Hub' page (please see below)

On the left hand side of your account, you will see the side menu in which you can select 'Messages'.

You will then see a list of all previous conversations with clients

By clicking the 'New' button

You will be able to select which client you would like to send the message to and also type the message out.

 Then just hit the 'Send' button.