$ diskutil list
and it returns something like


/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk0
   1:                        EFI                         209.7 MB   disk0s1
   2:                  Apple_HFS Macintosh HD            999.9 GB   disk0s2
/dev/disk1
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *4.1 GB     disk1
   1:                 DOS_FAT_32 GENTOO LIVE             4.1 GB     disk1s1

Now unmount the disk using diskutil replace the disk number with the id of your usb stick

$ diskutil unmountDisk /dev/disk1
$ dd if={ISO_IMAGE_HERE_} of=/dev/disk1