Thank you eureka, now I just get 12 warning and imx_usb has been created. However, I guess I am unable to find my kindle in usb_download_mode. Do I have to put any special commands so that the kindle is used in the right way on my mac (of course I hold down the power key until the lamp goes of, then hold down menu key, then release the power key and finally release the menu key)?
by just using imx_usb, I get:
$ ./imx_usb fastboot.bin
15a2:0052(KindleTouch) bConfigurationValue =1