I guess you haven't read the manual, did you?
When logged in as reseller or admin, first step is creating a new customer. A customer can be assigned webs which are created in a seperate step either with the main menu or within the second tab of the edit customer form.
Within a web you can then add email accounts. You can mark one email account per web as "administrator" who can login via ftp to the web's home dir. The other accounts only have access to their own homedirs which are located in the web's users subdir.