Currently Being ModeratedOct 26, 2012 2:05 PM (in response to tarmar)
Have you already shared your song to iTunes?
You will only see it in iTunes, if you create an audio file from it using the menu command "Share > Send Song to iTunes". This will add the song to a playlist and you can access it from Keynote using the Media Browser.
Or you can save your GarageBand Project with an iLife Preview, then you can see the GarageBand project in the GarageBand Section of the MediaBrowser.