The chat interface is drawing your attention to a communication from your browser, which is asking you if you want to allow the browser to use your webcam and microphone.
On Chrome, this is a bar that runs right below the address bar and reads "WeSpeke wants to use your camera and microphone. Learn more." and offers you the option to Deny or Allow. You must Allow to take part in a video chat.
On Firefox, you will get a drop down menu that asks which audio and video equipment you want to use for your chat.
(HINT: If you didn't know to address these problems before you clicked on this help desk article and the bar or box disappeared on you, you can just cancel the chat you're in and start a new one—the bar or dialog box will reappear.)
See I get an error when I try to chat! if you are still having trouble.