Are you on windows or mac ?
Do you have quicktime installed on your system?
iTunes music includes digital rights management software that will keep you from using it in a video project. (That's why most people buy music for use in videos from Amazon.com, which doesn't have this DRM software.)
To get around this, you will need to burn your iTunes music to a CD and then rip it back to your computer. I also recommend converting the file to a WAV for best performance.