If you can’t play songs you’ve bought online, the problem almost always boils down to one of these three things:
You don’t have QuickTime 6.5.1 or later. You need this version of Apple’s QuickTime music-and-movie-playing software if you hope to use your store-bought songs in the other iLife programs, like using a song as a background in an iMovie movie.
To find out what version you have, open System Preferences, click QuickTime, and click the About QuickTime button. If it turns out you don’t have the latest version of QuickTime, stay right there in System Preferences. Click View → Software Update, click Update Software, make sure you can get online, and then click Check Now. After a moment, you’ll be offered the opportunity to download the latest, greatest version of QuickTime.
Your permissions are scrambled. An amazing number of mysterious glitches arise because the permissions of either that item or something in your System folder have become muddled—that is, Mac OS X’s complex mesh of interconnected Unix permissions settings. Music-store songs not playing back is among them.