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Info here :http://www.ehow.com/how_4827753_install-yahoo-chat.html

Users can navigate a broad list of free topical Yahoo! Chat rooms by double-clicking on the files next to each subtopic in the Yahoo! Chat menu. Once you have located a chatroom you would like to enter, select “Go to Room.” 

Your Yahoo Chat window will begin loading in Yahoo! Messenger. Once the animated bar indicates the chat has loaded, click “Enter Chat.” 

Before users can enter a ">Yahoo! Chat, they must enter a series of random digits and characters. This helps shield users from computer-operated spambots from entering Yahoo! Chat. Enter the characters (as they appear in the image) in the text field in the shaded window, then click “Submit.” 

After successfully entering the characters in the Yahoo! Chat, the chat room will load and you can begin conversing with new Yahoo! Messenger users from down the street to around the globe! 

Source http://im.about.com/od/advancedimfeatures/ss/yimchat_4.htm