Here is some help for troubleshooting login of Messenger 9.0 beta:
http://help.yahoo.com/l/us/yahoo/messenger/messenger9/messenger/mstanoconnect.html
Please note at the bottom of the page is a Contact Customer Care. Use that button if none of the troubleshooting steps resolves your issue. When you click that button a web form will appear in your browser. Fill it out carefully, explain your issue, give details of your system such as OS, manufacturer, make and model, amount of RAM, processor, and the browser you use. Customer Care will get back to you usually within 24 hours.
The link below contains some extra steps on removing Yahoo! Messenger a different way, and then reinstalling the application:
http://help.yahoo.com/l/us/yahoo/messenger/messenger9/messenger/mstareinstallxp.html
http://help.yahoo.com/l/us/yahoo/messenger/messenger9/messenger/mstaerror.html
I hope this helps get you up and running, good luck and have a nice day.