To create a mailbox, you should do this :

1)  Login to your control panel.  http://www.youdomain.com/cpanel

2) Click "mail" in the web panel.

3) Click "add/remove accounts".

4) Click "add account" (bottom of page).

5) Enter the name of the account in the email field (If you wish to add an Alias like first.lastname, you can do this later.  It cannot be the username though.  Should not contain any special characters.)

6) Enter the password for the new account. (User can change password by logging into webmail.)

7) Optionally modify the default quota for the mailbox. (Use 0 for unlimited).

8) Click "create" and a screen confirming the creation will appear.

9) Click "go back" to return to the list of accounts and the new account will be listed.