If you have a Universal or Infinity plan, you will need to purchase the Reseller FTP option from myCP -> Order Account Add-Ons. Once you’ve purchased the Reseller FTP option, a unique FTP login will be created for each of your domains. The FTP login is created automatically, if you need a custom FTP login/username, you’ll need to upgrade to a Custom Pro plan.
If you have a Custom Pro plan, Additional FTP Access can be ordered from the myCP -> Order Account Add-Ons page.
If there is anything not covered here, please log into myCP and open a ticket.