The same as adding video in the prgroam, either use the Media Pool and navigate to the folder where the file is and drag it onto the timeline, or drag it direct from Windows File Explorer to the timeline.
The audio must be in a supported format eg mp3, wav, ogg, flac etc, supported formats are listed in the PDF manual available under Help.
The tracks in Movie Edit Pro (MEP) are universal so you can place them on any track you wish, however I would advise using track 8 or a higher track number so as not to interfere with any effects, collages etc, that you may be using.
The question wouls suggest you are new to MEP therefore I would suggest you watch browj2's tutorials on getting started with MEP in the Tutorial section of the forum starting with this one.