To check your webmail you can go:
http://www.youdomainname.com/webmail OR
Login to your control panel, under, 'Mail' --> 'Manage/Add/Remove Accounts' and click the 'Read Webmail' beside the account you wish to check.
Remember for the emails you create your login is the full email address, ie:
user@yourdomainname.com
If you just logged in your control panel and click on 'Webmail' you are actually checking the account you logged in with.