What version of Premiere Elements, what computer operating system? Is this m4a from iTunes?
Generalizing for now without details, please take the m4a into iTunes and convert it there to the WAV version. Then import that WAV version into Premiere Elements project using Premiere Elements' Expert workspace Add Media/Files and Folders/Project Assets from where you drag the file to the Timeline audio track.
The how to for the iTunes conversion from m4a to WAV version can be found
Please consider, supply more information, and then we can decide what next as indicated.
Any questions or need clarification on any of the above, please do not hesitate to ask.