Break your work down into smaller parts.
First, get the joystick demo working. You have the library. You have the example in USBHIDJoystick.ino
Have you tried uploading that and seeing if it works with your joystick?
After you get the joystick reading working, then tackle the next step - your motor driver. Does the manufacturer provide any tutorial or examples with it? Try to get it working without your joystick being involved. After you get the motor driver working, THEN work on combining the 2.