I think I still have an MDI box, loom and green banded iPhone lead at home somewhere you can try if you want. I couldn't get it to work. These RNS510s are pretty crap imho, too complicated, too expensive, need firmware updates to do anything useful etc etc. The blight of modern cars = electronics.
Anyway, I had the same problem with the SD card. Firstly, what revision is your RNS510? Some of them can only read 2GB cards (Max) and it has to be formatted FAT32. Later units can read 4GB and higher cards.
In iTunes, you need to choose your music, right click and choose "Create MP3 version". The files are then ready to put onto your SD card.
Hopefully, your iTunes is on a windows machine. If it's on a Mac, that's yet another problem because Macs can't see windows formatted discs. I had Windows running in a VMWare fusion session on my Mac, so I could copy files over.
It's a right ball ache. An aftermarket Apple Car play stereo is probably the best option but they are still in their infancy. No wifi, no Bluetooth etc. I'm waiting for the technology to improve before taking the plunge!