You can view and answer all your chats from the Unified Chat Desk. Log in to the chat desk. All chats in your inbox are displayed on the left. Just click on a message and type your reply in the chat reply input box at the bottom of the screen. Hit enter or click send.
If you have admin rights to your Clickatell account, you can also navigate to Chat Desk from within your account. On the left-hand navigation, go to Touch > Launch Chat Desk.