Are you trying to transfer the GarageBand projects or the rendered audio files? What version of GarageBand do you have on your iPhone?
I do not think you can share your GarageBand projects from your iPhone to GarageBand on your Mac, unless you have iCloud, afaik, only the audio files.
How did you transfer the song?
To share from an IOS device the Projects or audio files, see: Share GarageBand songs