Here's how to add a custom status message:
→ In the main Messenger window, Click inside the box below your name, where it says "share a status message".
→ Type your message, press Enter on your keyboard and your status message is set.
☼ Your custom status message now appears next to your Yahoo! ID in other peoples' Messenger Lists.
☼ You can create up to nine custom status messages. To select a different one, click the down-arrow and select another.
☼ Please note: Older custom status messages are deleted as you create new ones.
☼ To remove a custom status message, click the down-arrow and select Clear My Status Message.