You wouldn't be able to import M4P files- these are iTunes copyright protected files. Otherwise, if they aren't copyright protected files, make sure you are importing them from their actual location on your hard drive - the files aren't located in iTunes - it's just another program that plays the files.
What Morbotron said. Also, even if your files are not subject to any kind of copy-protection/DRM scheme, Movie Edit Pro doesn't support Apples M4A format, which they use to store unprotected AAC content. Check the documentation of Movie Edit Pro for supported formats for audio import. You do have a few options to choose from, e.g. WAV, MP3 and audio CD.
Here's another tip: If you used iTunes to rip your own audio CDs onto the computer and now you want to use the files in Movie Edit Pro, you will not be able to do it with the M4A files that iTunes creates by default. However, you can actually configure iTunes to create MP3 files. That would work. Then again, if they're your own audio CDs, you might want to prefer importing directly from the CDs into MEP in the first place, in order to avoid additional quality loss from repeated, lossy audio compression...