I have a Macbook 13 Early 2011 and it did not work with the syslinux partition on USB as originally described here, but rather was similar to the above comment.
I created a small FAT partition at the end of an internal HD that I installed in place of my optical drive.
I then used the dd command to put the syslinux image there (partition to partition rather than disk to disk looked like: sudo dd if=/dev/disk2s1 of=/dev/disk1s4 bs=1m ). Then copied over the IMG file as instructed and changed the cfg file to reference the image.
The solution then worked as described, booted into the menu and I was able to update the SSD firmware on a crucial M500 480GB
Thanks to the author and commenters!
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