You go to the "User & Email" tab of a web site in ISPConfig and simply create a new user there, e.g. web1_john with the password john and the real name John Doe and the email address email@example.com
When you use the ISPConfig webmail package, then the login is firstname.lastname@example.org
with the password test.
When you use an email client, you have to use web1_john:test (and make sure you enable "Server requires authentication." in your email client, otherwise you won't be able to send emails!).
It's important that you first send an email to that user (because his mailbox will be created when the first email arrives) because otherwise you'll get an error message when you try to login via POP3 saying that the mailbox doesn't exist.
Also have a look at the ISPConfig documentation.