Trouble using the DFU programmer for HID w/Arduino Uno R3

I am trying to use different firmware on my Arduino UNO R3 so that it can act like a keyboard. The computer I'm using with it is a MacBook Pro with OS X Mavericks. It seems the way to do this is with the dfu-programmer script. But I am having trouble. I can put the Arduino in DFU mode and then I run:

$ sudo dfu-programmer atmega16u2 erase

But this never ends. I've let it go for quite a while 10+ minutes and nothing happens. (Then I Ctrl-C to exit.) I have also tried running other commands and everything just goes forever like this.

It seems to know the device is there, because if I change the command to something other than atmega16u2 then I get a message "no devices present," but when I have atmega16u2 in the command I get this never-ending situation.

What is going on? Am I missing something really trivial?