If Messenger gets stuck when you try to sign in, follow these steps:

1. Quit Messenger.
2. Open the Finder window (click the Mac icon on your dock).
3. Click your user name on the left side.
4. Open the Library folder.
5. Open the Caches folder.
6. Open the Yahoo! Messenger folder. Drag files in that folder to the Trash.
7. Click the "Back" button twice until you are back in Library.
8. Open the Preferences folder.
9. Find the files that start with "com.yahoo.messenger." Drag them to the Trash.
10. Start Messenger again.

You should be able to sign in now.