Warning. Follow these instructions at your own risk. I am not responsible when you use dd to blow away your kitty pictures.
Download the FreeBSD 8.1 memory stick image from a FreeBSD repository. I picked one from here.
Plug in your USB stick. OS X will mount it.
You want to unmount the volume while keeping the drive available. To do this, open Disk Utility, click on the name of the mounted volume, and click the Unmount button.