Once you select a username, you should see a "Talkinator" chat window above. Note that this is a public chat area; whatever you enter will be visible to anyone else entering this chat room (though it will probably disappear soon; this works only for people who are in the room at the same time).

Tips: Use the buttons at the top right to change your username, show the list of users online now, or change the font size. (Roll over the buttons with your mouse pointer to see what they do.)

Talkinator is still in beta testing mode. If it doesn't seem to work, try it from a different browser or computer, or see Talkinator.com or Talkinators.tk for more ideas.