The PWM frequency is very close to 1kHz (it uses the same timer to measure ms). The percentage represents the duty cycle, so at 100% you should not see any PWM.
External power does need to be connected to run the motors (i.e. not just USB). If you have power connected (e.g. batteries), you can check connection by attaching the motor directly to the other, free, power pads on the Crumble. You may have already done this though?