Made some progress, I think... I moved SPI.begin() to before the SPI.setBitOrder(), SPI.setClockDivider(), and SPI.setDataMode() commands. Now the LED does light up and I get different output, though still not correct:
Z Accel (g): 0.64
Z Accel (g): 0.64
Z Accel (g): 0.48
Z Accel (g): 0.48
It seems to only give one of those two values at a time depending on how I tilt the IMU. Better than nothing...