Crystalbuntu USB Install from Mac

I have an ATV1 with Crystal HD card installed.

I'm trying to create a Crystalbuntu USB drive using a mac running OS 10.8.3.
I've been following the instructions here http://wiki.xbmc.org/index.php?title=Cry...l_from_Mac

I've downloaded the usb.img file which is residing in my Downloads folder, however I can't seem to get past the second stage:

Open Terminal, navigate to the downloaded file, and enter the following command followed by return:
gunzip USB.img.gz


Could somebody walk me through the steps to navigate to the file?

Many thanks