To enter the ChatBox just simply scroll to the bottom of the screen where the Who's Online information is located. There will be a link to "Join the ChatBox". Click on it, it should send you to a different webpage and you may need to click on "Login" in order to connect.
After that, it is simply just typing what you want to say within the text box and hitting enter.
A few commands you may use is the "me", "exit", and "abs" codes, followed by the forwardslash (/).
/me -- In the beginning of a sentence, this command will be replaced by your username (ex: /me eats a delicious pizza" will display "Nickname eats a delicious pizza")
/exit -- Disconnect from the Chatbox. You can also precise the reason that will be visible for all members connected (ex: /exit reason)
/abs -- Send a message noticing that you're absent. You can add a reason that will be displayed in the message and visible by all members connected to the Chatbox (ex: /abs reason)