CS6 works with Itunes, the format I tried was m4a, mp3 also works. Try playing the music file in quicktime and or windows media and see if it plays. if it plays then the codec is installed, which it should be anyway, if you can play it in Itunes.
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What version of iTunes do you have. Initially, iTunes used DRM (Digital Rights Management), and their music was not directly usable by other applications. That was relaxed at a point in the past, and now, with newer versions of iTunes, things are easier, especially if one also has a later version of Apple's QT Player installed on that system.
In the old days, one would create a Playlist in iTunes, populate that with the necessary iTunes songs, then Burn a CD. From there, one would use an audio-editing program, such as Adobe Audition, or maybe the free Audacity, to rip the CD and Save the songs to something like PCM/WAV (Uncompressed) @ 48KHz 16-bit.
Also, one BIG consideration is the distribution of the SlideShow. All music (except original performances of original music) is covered by © laws for the region where one lives. Be sure to check those laws, if you are going to do anything but play the SlideShow for your own enjoyment - even then, it might be best to check.
Because of the © issues, I almost always use Royalty Free Music in all productions, whether personal, or for public display.