What I think happens is that you need to 1) power up laser 2) insert usb plug into Mac and 3) open Lightburn. If you have Lightburn open and then power up, the laser will not show in Lightburn, although you can find it in /dev. Fix this by quitting Lightburn and then open it again.
To check the USB ports in /dev, open terminal
MacBook$ cd /dev
Macbook$ ls cu*
you should see
cu.BLTH
cu.Bluetooth-Incoming-Port
cu.usbmodem1234561
the last one indicates you Mac is seeing the laser.