There is no int0 on the Sparkfun board. It's INT1 and INT2.
Using the interrupt feature of the IMU is optional. If you do want to use it then you need to uncomment line 31:
//int interruptPin = 2; // Setup pin 2 to be the interrupt pin (for most Arduino Boards)
and also line 73:
//adxl.setImportantInterruptMapping(1, 1, 1, 1, 1); // Sets "adxl.setEveryInterruptMapping(single tap, double tap, free fall, activity, inactivity);"
// Accepts only 1 or 2 values for pins INT1 and INT2. This chooses the pin on the ADXL345 to use for Interrupts.
// This library may have a problem using INT2 pin. Default to INT1 pin.
That line is currently set to configure INT1 on the IMU board to be used for single tap, double tap, free fall, activity, and inactivity events. So the short answer is that you should connect INT1 to pin 2 on your Uno. If you want to use INT2 for any of those events then you can just change the relevant arguments from 1 to 2.
You also need to uncomment line 84:
//attachInterrupt(digitalPinToInterrupt(interruptPin), ADXL_ISR, RISING); // Attach Interrupt
that will cause the ADXL_ISR function in your sketch to be called whenever the interrupt is triggered. If you wanted to use INT1 and INT2 then you would need to add another similar line but configured to call another function, which you would need to write, using ADXL_ISR() as a reference. Note that you can only use pins 2 and 3 on the Uno with attachInterrupt(). For more information see:
https://www.arduino.cc/en/Reference/AttachInterrupt