There are three ways to upload flat files depending on your security policies:
Download FileZilla - Free (S)FTP application for MAC or PC. Contact Support@rightoninteractive.com to retreive (S)FTP credentials if needed.
Download/Install another FTP application. You will need to add your external (S)FTP credentials to an External Segment in ROI.
Use Windows Explorer if the above two options are not available - See screenshots below.
Any files dragged into this folder will be uploaded to FTP. Contact Support@rightoninteractive.com with additional questions.