Dfu says "no device present"

After trying to update firmware for Uno r3 (following directions on http://arduino.cc/en/Hacking/DFUProgramming8U2/), the Dfu says "no device present" when I try "sudo dfu-programmer at90usb82 erase" in Terminal on 10.9.1 OSX. Please Help!!!

This is what terminal said :

Last login: Mon Dec 30 15:08:53 on console
appmatoair:~ Zachzhao$ cd /Users/Zachzhao/Desktop/Arduino.app/Contents/Resources/Java/hardware/arduino/firmwares/atmegaxxu2/arduino-usbserial
appmatoair:arduino-usbserial Zachzhao$ sudo dfu-programmer at90usb82 erase
Password:
dfu-programmer: no device present.
appmatoair:arduino-usbserial Zachzhao$