If you are still using Yahoo! Messenger version 5.5 and 5.6 as of today you will not able to. Yahoo! Messenger team making them retire so they will not support it.
If you do not want to update to latest version you can use
Yahoo! Messenger for the web.
Download Yahoo! Messenger
Yahoo Messenger 8.1 (Latest stable version for now)
Yahoo Messenger 9 (Beta)
Yahoo! Messenger for Vista (Alpha)