To use SMS to your customers click on the SMS card:
You will be shown the messages you have sent or received grouped by customer
Green highlighted messages are new ones you've not yet read.
Clicking in to View lets you see the full message thread and send and receive messages and use any templates you have setup.
Click on the New button in the top right to send a new message to someone
You can either free type the number in the Recipient box or click on Contacts to send a message to someone from your address book. You can now type in your message or click on the Templates button to use one of your own
Now click send and the message will be sent right away