Addon Domains
Again the page is self explanatory. Just to point out that where your package means you can host more than one domain your first domain will be the main account, and any other domains will be set up under that main account using the same resources.
Set up the directory in the same name as the domain to avoid any confusion!
FTP
Your FTP address will be ftp.yourdomain.com, with the user name and password that you were given for your account.