To play back DSD files using your iOS device via Onkyo's Player you will first need to download the application to your phone. It is a free app, however, to use the "HD" function within the app, you need to purchase that section for around $10.
To add the files to your device follow these easy steps.
Connect your device to the computer
Navigate toward the file sharing section for the device in the iTunes window
Find the ONKYO HF app in the list of file sharing apps
Click 'Add files" in iTunes
Select and add the files from the directory.
To explain this easier, you may also use this short and handy video I made!