Hi Pepe and fancygirlie
I'm a Mac user too and have successfully downloaded and used Switch.
You need to be familiar with the file structure of Mac. You have a "User" folder and within that you'll find your username, documents, movies, music, pictures, library, etc.
Your music is stored in the music folder and on my machine it is sorted by artist name. When in iTunes you need to be aware of whether or not the song or album is classed as a compilation. If it is, then when navigating into the music folder it may be stored under compilations not the artist name.
If you do not wish to use iTunes to listen to your songs you should be able to play via the "finder". I would suggest change the folder view to column view that way you have the song title on the left and the actual file on the right and you should see the play arrow.
For converting the files to another format or bit rate I would suggest making a copy of the song first (or choosing a new folder as the destination folder) so that if you change your mind you still have your original.
Hope that helps!