For writing the usb image files, likes the one mentioned at ftp://ftp.freebsd.org/pub/FreeBSD/releases/amd64/amd64/ISO-IMAGES/10.3/, to usb drives on a mac, see this link https://people.freebsd.org/~seanc/freebsd/create-memstick-osx/ .
The usb images are differentiated from the normal ISO files by the presence of “memstick” in the file name.
Cheers
Amitabh Kant 