Originally Posted by Poetcop
Your real challenge now is to get talking to your Kindle in fastboot mode, which you can test by running a simple command like "fastboot getvar serial" (gets the serial number of your Kindle).
If using fastboot for linux, be sure to run it as root (or prefix the command with "sudo "). If not root, it will not see any USB devices in fastboot mode (< waiting for device >).
If using fastboot for windows, be sure to install the lubusb-win32 device drivers, or it will not see any devices in fastboot mode (< waiting for device >).
The Mac version probably needs to run as root too...