Step 1 : Find the usb key identifier
First, you need to find what your usb drive id (/dev/disk*). To find out, you will need to use this command :
user@mac $ diskutil list
If you are not sure which device is you key, type the command before and after plugin it. That way, you will be able to see which device was added.
Step 2 : unmount the usb key
Now, unmount your usb using the identifier you just found.
user@mac $ diskutil unmountDisk /dev/diskN
Step 3 : convert the iso into a .img file
It appears that it is not possible to create a bootable usb directly from an ISO. Therefore, we need to convert the iso and create an .img file.
user@mac $ hdiutil convert -format UDRW -o /path/to/target.img /path/to/image.iso
The created file will have the .dmg extension for no reason (Thank you OS X) so I suggest you to rename it
user@mac $ mv /path/to/target.img.dmg /path/to/target.img
Step 4 : Make the bootable key
The only thing left to do now is to copy the content of the .img on the usb drive.
Note that in the following command, an 'r' has been added to the identifier ot the disk
user@mac $ sudo dd if=/path/to/file.img of=/dev/rdiskN bs=1M
Step 5 : You are good to go
You can now boot on this usb key !