It doesnt understand the file format?

Im tryin to rip the vocals from a song in my itunes library using audacity. I watched a tutorial on youtube on how to use audacity to rip the vocals from the song. It said to drag the song to audacity from its place in my library but when i did this, audacity said the file format wasnt understood. Im really confuzed but I really want to be able to remove the vocals from this song. Any help would be GREATLY appreciated.