How can I copy ITunes music to Photoshop Elements11 slideshow?
Use the PowerPoint command Insert > Audio > Audio from file, and navigate to where the audio file is located.
If you don't know where the file is located, right-click the track in iTunes and choose Show in Windows Explorer.
Make sure the song you want to use is in MP3 format. If it is in something else, make an MP3 version, per this document:
iTunes 11 for Windows: Save a copy of a song in a new file format