First, you need to know which drive is your SD card.
Open a Console and list your current drives and partitions with:
$ diskutil list
This command should output something similar to this
In my case, disk0 is my hard drive, and disk0s1 to disk0s5 are my partitions.
Now plug in your SD card, and type again:
Now that you know your SD card drive, go to where you extracted Lakka, and flash the card.
Please note that dd is a very dangerous command: if you give it the wrong drive identifier, it could erase your hard drive instead of the SD card!
$ sudo dd if=Lakka-*.img of=/dev/rdiskN
Where diskN is your SD card drive.
It should take a few minutes, wait for the prompt. Once done, you can unplug your SD card and proceed to the next step.
If you get this error:
dd: /dev/disk4s1: Resource busy
You have to unmount every partition of your SD card, this can be done with:
diskutil unmountDisk /dev/diskN
And then you can retry the dd step.