Update 8/3/15:
Kindle Freetime no longer works with any 3rd party music players. We now recommend customers use Screen Time by Screen Time Labs. More information on Screen Time Labs can be found on their website.

Previous 12/24/13: Many customers have emailed support with frustrations over trying to listen to music in Rocket Player with Kindle Free Time. The Kindle free time app works in its own partition hard drive/storage space, which is why even when you have music, you can’t see it in Free Time.
Try the following provided by Rocket Player listener and supporter Libby H:

1) Music that is desired to be accessed by Rocket Player must first be downloaded to an external hard drive or computer.
2) Open Kindle Free Time
3) Connect the Kindle to the Computer/Hard Drive via USB
4) Open the Kindle Music Folder
4) Transfer (copy) the music files from the computer/hard drive to the Kindle Music Folder