There is another way. If you have a Dropbox account and the app on your iPad. Download the song to Dropbox from your computer. Open Dropbox on your iPad, navigate to the file. In the right hand upper corner is an arrow. You can email the mp3 from there or scroll down the drop list ,copy it and attach it to your email in the email program.
A work around I know, and I wish apple would make it easier, but I love my iPad and how it syncs with everything .