Stuck waiting for device in fastboot

Trying to follow the steps to boot into TWRP using this method:
http://forum.xda-developers.com/smar...-easy-t3107501

I am able to boot into fastboot and my screen shows "Running Fastboot" but when I issue the command fastboot boot "<filename>" I just get < waiting for device > on my PC.

Any idea what is getting stuck? I factory reset the watch hoping that would solve the issue but no luck. ADB works fine.