Archived - GarageBand: Tip - Exporting Songs to iTunes
Make sure you close any open dialogs in iTunes, if iTunes won't import a song you made in GarageBand.
When you export a GarageBand song to iTunes, the song may not appear in iTunes, and you might see a "time out" alert if there are open dialogs in iTunes. Make sure to close the Preferences, Get Info, and Import windows in iTunes before exporting from GarageBand.
Also, make sure you don't move or rename iTunes.