I have just bought a car with a JVC WMA MP3 KD-9721 CD/Radio.?

It has a USB port so I plugged my phone in to it (a Nokia Xpress Music) but can't work out how to get the music on my phone to play through the CD/radio, when I plug it in it says 'check' on the cd/radio display? Having never had a car radio with this facility I don't really know what Im doing! Any suggestions please?