1) Connect the Midi Fighter to your computer
2) Flip it upside down and look for the small hole on the back surface
3) Use a paper clip or something similar to push the white button sitting below the hole. Make sure you press down on the switch, hold for 1 second then release slowly.
4) If it works you should see a blue checker board pattern on the arcade button LEDs.
Remember this can be a little tricky so try again if it doesn't work first time.