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You have to have the song that you want to use in your music library (so when you launch the iPod application, the song has to be there).
 
The song must be put into your iTunes library from a CD or downloaded off of the internet. If you buy it from the iTunes Store it's protected from being used in your video because it's technically copyright infringement.
 
Briefly tell me how >__> I can't find the way to add it.
Initially start a new project and pick a theme and select a clip.
Then click the insert button. It looks like an empty square with an arrow pointing down. It will default to video. At the bottom of the screen you should see Video, Photo, Audio. select Audio and pick something from your play list.
 
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