Board doesn't turn on (the green power LED doesn't light up)
If you're using an older USB board, make sure that the jumper (little plastic piece near the USB plug) is on the correct pins. If you're powering the board with an external power supply, the jumper should be on the two pins closest to the power plug. If you're powering the board through the USB, the jumper should be on the two pins closest to the USB plug.