You have to look for the motor control chip on the robot's board, located in the base. Then apply 3v to each pin on the chip to map out which pin controls which motor, and in which direction. Once you have this information, you can solder wires between these pins and the Teensy. Inside the code, you can set these pins to High or Low when you send the Teensy serial data, activating the Robot's motors. Reading through the code I posted should put you on the right track.