You cannot transfer song files from iPod to computer using iTunes. However, there are third-party methods and utilities that can transfer from iPod to computer. Since you are going from Windows PC to Mac, it will work, because Macs can read the Windows disk format (the opposite is not true).
If you do a Google search on something like "ipod transfer," you should get some links. Once the song files are on your MacBook Air, just add them to your iTunes library.