Flashing Linux like Raspbian on a Flash-Card is pretty straight forward on OSX
First list all disks using diskutil after plugging in the card
diskutil list
Unmount the the found flash-card
diskutil unmountDisk /dev/disk2
If you got an ISO file instead of IMG, convert it first
hdiutil convert -format UDRW -o linux.img linux.iso
Write image to the Flash drive
sudo dd if=linux.img of=/dev/rdisk2 bs=1m # Raw interface is much faster
When done, eject the Flash drive again and you’re done
diskutil eject /dev/disk2