Hi there! The Roundcube for Joomla is an IMAP client, so in order to login there you need to first create an IMAP account, Roundcube does not do that for you, you will need to use another utility provided by your hosting service!
With that IMAP email account created, you can login to the component using as username the emaila address and the pass you set for the account! Joomla users can't login to the mail component if they don't have an IMAP account on the imap server you have setup for the Roundcube component!
For more details on using Roundcube, i also suggest you visit their site
http://roundcube.net/
Regards, Ovidiu