Like all the current crop of Unix derivatives, FreeBSD is a plug and play operating system, which means it should detect new hardware.
Unix treats everything as if it was a file or directory and USB drives are no exception. To view the contents of the USB drive it must be mounted to a directory in the Unix file system.
Firstly plug in the USB drive. FreeBSD should pop up a screen displaying a message that the new device has been recognised. Different Unixes refer to different devices (and sometimes the same device) using various obscure names. Sometimes it's a bit of a guessing game - the file representing the drive will be inside the /dev directory, alongside all the other hardware reference files.
One of the most common names for an external USB drive under FreeBSD is da0s1 and this is the one which works most of the time for me.
Firstly create the directory which the USB drive will be mounted to..
Then run the command to mount the drive using the MSDOS (FAT32) file system..
mount -t msdos /dev/da0s1 /mnt/usb
And that's about it. The files should be accessible from the /mnt/usb directory