Running Chromium OS from bootable USB on MacBook?
From my experience you are best downloading VirtualBox and launching it as a full-screen virtual appliance. Apple's EFI doesn't like to load USB drives that aren't OS X or Windows 7 installers. I have run into this same issue trying to run Ubuntu and Haiku from USB. If you have an optical drive you are better off burning the ISO to DVD and booting that way. You could also checkout rEFIt at your own risk as an alternate boot loader. 

By the way it is normal for OS X to not recognize the format of the USB you created. That is because it is probably not in HFS+, NTFS or FAT. Not that it makes it any easier to boot.