Ardumoto Help?

I recently bought an ardumoto (about 4-5 weeks ago) and I’m kind of lost. I’ve powered it with a USB connection to my computer and a 4x -AA battery set in the barrel jack of the Arduino. I’m trying to use it to power a Tamiya Twin Motor Gearbox, and the LEDs (yellow and blue) are lighting up depending on the PWM and DIR pins I turn on, but the motors aren’t turning. Am I doing something wrong?

(It’s connected to an Arduino UNO)

I read somewhere that if I bent the VIN Pin out, I could drive the Ardumoto directly with 18v (So I don’t fry the Arduino), but that made me even more confused because I’m not sure how the Ardumoto receives information from the Arduino, and I wanted all of the details before I tried anything major, like cut a pin.