Tracks purchased from the iTunes Store are AAC, not MP3, so if you need MP3, you'll need to convert the tracks. iTunes can make a copy as MP3; see:
or for iTunes 11:
or you can use a third-party audio converter. If you use iTunes to convert the tracks, drag the tracks from the iTunes window to the flash drive's window, or locate the tracks in the iTunes folder (usually in the My Music folder in your user's Documents folder) and drag-copy from there.