The MP3 Uploader will churn through your iTunes library and eventually detail the number of songs and playlists it found that aren't on your Cloud Drive. Near the bottom of the window you'll see the amount of storage you have available as well as how much more storage you'll need to upload all your compatible music (along with a Buy Additional Storage link). Compatible music includes unprotected MP3 and AAC files. The service doesn't support WAV, AIFF, Apple Lossless, FLAC, or Ogg file formats. Similarly you can't upload tracks greater than 100MB, nor ringtones, podcasts, audiobooks, and, according to Amazon, "other non-music audio files."

If you have a fairly small music collection that can fit in 5GB of storage, click the Start Upload button and all of your music will be uploaded to the service. The estimated time this will take appears in the window's bottom-right corner. If, like many of us, there's no way all of your music will fit, click the plus (+) button next to the All Music entry. This reveals Playlists and Artists entries with plus buttons next to them.

Click the plus button next to one of these entries, and you'll see your iTunes playlists and artists, respectively, with a check mark next to each one. If you click the plus button next to a playlist, you'll see the contents of that playlist, but you won't be able to upload individual tracks from that playlist--it's all or nothing. However, you can select individual albums and tracks listed under the Artists entry. To uncheck everything in these lists, just uncheck the main Playlists and Artists entries.

You're not confined to uploading just tracks from your iTunes library. If you'd like to venture to another location on your Mac, click the Browse For More Music entry at the top of the window. A Select a Folder window appears, from which you navigate to tracks you'd like to upload.

Once you've selected the tracks you want to upload, click the Start Upload button at the bottom of the window. A progress bar appears here. To pause upload, click the aptly named Pause Upload button.