Chat by typing in the message area at the bottom of the screen. Just hit return at the end of your message to send it. Join new rooms by selecting the room from the Rooms list and clicking the Join button.
Whisper sends private messages to a single user.
Select the user's name in the Users list, click the Whisper checkbox, and anything you type in the message area will be sent to just that user when you hit return. If after you push the Connect button it changes briefly to Disconnect and then back to Connect, try using a different Nickname. Disconnect by clicking the Disconnect button. If you leave the chat page for any reason, you will be automatically disconnected.