I had a problem with the fastboot command not working on Mac. Turns out the binary files needed to be included in the $PATH variable. Here's the info I found that helped me resolve this.
On Mac OS, you'll need to add the "Platform Tools" directory to your $PATH variable. That means opening up a terminal, typing "sudo nano /etc/paths," entering your password, and adding the path of the "Platform Tools" directory to the bottom of the file. Then hit "control+X" on the keyboard, "y" and then "enter" to save, and you're done. Then you just need to run "flash-all.sh" from the terminal.
Rather than adding the path, I just copied the adb and fastboot files to the usr/bin folder since that path was already in there.