[BrianG]

You are using an external BEC. Assuming it is connected right (sounds like it is), did you disable the MM's BEC?

I assume you are following these instructions (particularly the bold):

1. Solder a high quality battery connector to the ESC.
2. Mount the ESC and motor into the car.
3. Plug in the 3 motor wires to the 3 motor wires on the ESC.
4. Plug in the ESC Rx lead to CH2 on your receiver.
5. Make sure the ESC’s switch is off.
6. Plug in a battery.
7. Holding full throttle on your transmitter, turn the switch ON.
   1. After a few seconds you’ll hear multiple tones and the red LED will come on.
   2. Now hold full brake and after a few seconds you’ll hear multiple tones and the yellow LED will come on.
   3. Now relax to neutral and after a few seconds you’ll hear multiple tones and ALL the LEDs will light up.
   4. A few seconds later the ESC will arm with a double tone and you’re ready to go!