FTP normally connects to the user folder. So if you have a domain example.com you would need to FTP to /example.com.
If you have a folder within example.com called images you would FTP to /example.com/images.
If you have created a sub domain then it will be a separate folder so you would FTP to it. e.g. FTP to /subdomain.example.com.
Depending n your FTP client you might or might not require the opening/and/or/closing slash.
Opinions are my own views and not the views of DreamHost.
Any advice offered by me should be acted upon at your own risk.