You will find your FTP login credentials in your "Sell Items" tab, on the right hand side you will see " FTP", click here to view your credentials.
Please keep in mind:
All files in an artists FTP account are removed after a 25 day period. This is to ensure there is ample space on the FTP servers for all artists on the platform to upload their content. The FTP system is not designed to be a storage hub for files, it allows the artist to quickly bulk upload content to their store that they plan on publishing within the next 25 days.