Short answer... you most likely can't due to DRM... more information by following the steps below
Please go back to http://forums.adobe.com/community/premiere_elements and type
in the area just under Ask a Question, type in your error message
You may now read all the previous discussions of your problem... be sure to click the See More Results at the bottom of the initial, short list if the initial list does not answer your question
As John says, the rights management software on many iTunes songs prevents them from being used in a video project.
This is why most people who use music in their videos get their music from Amazon.com.
Also, for best performance, convert any music to WAV files before you bring them into Premiere Elements. You can do it with the wonderful, free app Audacity.