Did you mean NMEA2000_CAN.h?
You should not touch any library code at all. If test example, you can take copy of it and modify that. If you need to change definition, you do it in your code before including NMEA2000_CAN.h - not modifying itself. Like this:
#define N2k_SPI_CS_PIN 10
#define N2k_CAN_INT_PIN 2
#include <NMEA2000_CAN.h>
Have you modified shield? As default shield has CS on D9! Also take care that you have exactly two termination resistor on the whole N2k bus.