Most iTunes music is copy-protected so that, if you try to use it in any video project (using pretty much any video editor) it will throw up an error code.
There are a number of ways to get around this. The easiest is to open the iTunes file in the free download Audacity and save the file as a WAV, the most efficient audio format to use in Premiere Elements.
Note, however, that you are still using copyrighted music. So some sites (like YouTube or Vimeo) may reject your video. Although that's usually pretty rare, unless you're selling the video.
Trying to add a little to Steve's answer, once you have a version of a music source without copy protection, you add it to the project exactly like you would add a video clip. You do not use the built in sound track tools. Once part of the project's "media collection" you drag it to any of the available audio tracks. The best is probably Audio 1 if you video clips are on Video 1. Once there on an audio track there are a wide variety of controls and tools available.