Thank you for your answers, Jura , MatrixGyal. I want to start chat with my friend. I can see "Chat Now" Button under the my friend's profile information , but it is always disable. Do you know how can I turn it on?

Welcome to Italki. Its quite easy to navigate. You can do what the italki moderator stated or you can also look at the bottom of your page and see a little task bar and you can click the green arrow and all the groups that you have joined would be there for you to chat. If you would like to chat a person individually, then you can go to the language exchange tab and look for people that are trying to learn your language and start speaking to them or add them as a friend so that you can get to know them better. I hope that helps!

Go up to the top right hand corner of this page and make sure it says Chat on.For more details see http://www.italki.com/answers/question/57961.htm