Making a bootable USB Drive on a Mac.

First of all, you need to format your USB drive ready for adding the bootable image to it. Start the Disk Utility, in Applications->Utilities. Select the USB drive you've inserted and format it per the options shown below.

$ hdiutil convert -format UDRW -o Fedora-18-x86_64-Live-Desktop.img Fedora-18-x86_64-Live-Desktop.iso

$ diskutil list
/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.1 GB   disk0
   1:                        EFI                         209.7 MB   disk0s1
   2:                  Apple_HFS Macintosh HD            499.2 GB   disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3
/dev/disk1
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     Apple_partition_scheme                        *17.4 MB    disk1
   1:        Apple_partition_map                         32.3 KB    disk1s1
   2:                  Apple_HFS Flash Player            17.3 MB    disk1s2
/dev/disk2
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                                                   *2.0 GB     disk2

$ dd if=./Fedora-18-x86_64-Live-Desktop.img of=/dev/rdisk2 bs=1m