I found the answer! A standard USB to 30 pin plug, EVEN AN APPLE CORD WON'T WORK. You have to use a special plug from Kia. Here's a link to see and buy one .
http://www.kiaaccessorystore.com/kia_ipod_cable.html
I went to my local Kia dealer today and bought one ($29). They had one in stock. Works like a charm. I can now listen to my iTunes library right from the phone, or I can pick music from the stereo controls. It also charges the phone.
My only complaint is the cord isnt long enough to reach my phone holder when plugged in. The parts guy advised me that it's best to have it unplugged when starting the car because the surge can damage the diode.
Heres the web page that helped me understand I needed one of these:
http://johnbiehler.com/2009/05/24/2010-kia-soul-car-tech/