If you've already setup the iPhone to use anonymous access to connect to your NAS, you may need/wish to change the settings under Services > Standard File Protocols > FTP in Frontview, to disable allowing anonymous access first and possibly change settings under Shares > Share Listing > Sharename > FTP as well. Then in the iPhone app when you see the list of your ReadyNAS Units, I think from memory you need to click the Edit button (top right of app - as in image below)
then you can select your NAS and change the FTP settings as shown in the image in chirpa's post.
You can see the detailed setup instructions (for initial setup of the iPhone app) at http://www.readynas.com/remote_iphone
. As you have already done the inital setup what you need to do is different (as described in the paragraph above), but you might find it helpful to have another look at the initial setup instructions.