You just download something like JFTP from jMethods (request a free license) which will run on any java equipped computer.
On the DNS-320 login as 'system administrator' and go to 'management' then select 'application management'. This will open a page that allows you to fill out the FTP server settings. Either use the defaults - recommended until you know what you are doing - or supply your own settings.
Save the settings and log out.
Install JFTP on your computer then connect to your DNS-320 by using its IP and the username and password you set up on the FTP page. You can then open Volume_1 and its directories and transfer files to and from your computer.