You can check your mail by signing in directly to your Business Mail account at http://mail.yourdomain.com (be sure to replace "yourdomain.com" with the domain name associated with your account). You'll be prompted to enter the Yahoo! ID and password you used to activate your email address.
You can also access your Business Mail account from your Email Control Panel (account owners only) or from your free Mail account:
If you're using the new Mail interface:

Note: If you're using the original Mail interface, click the email account you wish to view under "Mail Accounts" on the left side of the page.