Have MAC OSX 10.5.6 or aMAC Book slim? You have a problem entering DFU mode. Here is an easy fix from the dev-team blog:
1. You will need an account with ADC (Apple Developer Connection) this is free and takes a few minutes to sign up, you should read the terms and conditions carefully and you should only sign up if you are thinking of developing applications in the future -
Apple Developer Connection - Mac Dev Center
2. Download the disk image
“IOUSBFamily Log release for Mac OS X 10.5.5 Build 9F33” (yes, that is a “5” in 10.5.5 - this is a developer debug package of the USB kernel extension).
3. Install IOUSBFamily-315.4.1.pkg from within the disk image
4. Reboot your system.