Some people asked about Pidgin capabilities to join a chat room. I never used Chat Room before in YM nor Pidgin. So i tried to do some small testing on my Slackware box. First thing, click on Buddies > Join a Chat on Pidgin's main window. You will be asked what room you are joining (first screenshot). If you are unsure or you don't know what room to join, just click on the Room List and you will lots of rooms there (second screenshot). Just pick one of them and you will see an empty chat room filled with text from Yahoo Chat (third screenshot).
Go to the URL provided and entered the random CAPTCHA text there to prove that you are human
When you have entered the text, your account will be activated and you can start to chat on the room. Unfortunately, this steps will have to be taken each time you entered a room (even the same room in the future). I know it's daunting, but it's true. I don't know why they do this (i'm not sure YM users will have to do this).