Many if not most iTunes files are protected so you will not be able to open them in Audio Cleaning Lab. Some are not and you can export them as .MP3 or open them directly as AAC files in Cleaning Lab. It they are protected you won't be able to do anything with them at all other than recording them during playback through your system and then edit them with Cleaning Lab.
In iTunes right-click on the audio files name and select Get Info, Kind it will say Protected or Purchased
If it Protected then the DRM has prevented ACL opening the file and you cannot edit the file without breaking your license agreement for the audio files (or any DRM'd file).