As far as I can tell, the only way to access USB files is via an HTTP session to the SharePort port, 8181 by default. The SharePort app hides this detail, but otherwise provides no functionality you can't get from a web browser.
Unfortunately, the one thing I really wanted to do that D-Link claims works is copying files (such as photos) from the iPad to the USB drive attached to the DIR-505. The SharePort app provides no interface for this, and doing it through a web browser doesn't work (you can try to do the upload, but it just sits there doing nothing.) I can download files from the USB drive to the iPad using a browser other than Safari (I used iCab). Again, the SharePort app has no UI to download files - you can only "view" them.