If you are using windows on your computer , run media player with the kindle hooked to it by usb. Media player will see the kindle as a media device and you can drag and drop into the sync folder on the right hand side of the media player window.
You should see all of your music files (if you have them in the music library) and your kindle at the bottom of the folder menu on the left hand side. You can click on the kindle and see all of your media folders.
Hope this helps.
Edit: Once you have the mp3 files in the sync folder, click the sync icon at menu above the sync window your files that you want transferred are in.
Last edited by DustyDisks; 10-25-2012 at 10:08 PM.