The first step is get the gears back into the gearbox.

Start with the spur gear (looks like a spur) place it in the area shown.

Then place your sector gear (the gear with the half set of teeth)

Before you add the bevel gear make sure to get the anti-reversal latch and it's spring into place as shown.

Keep the anti-reversal latch pulled and then place the bevel gear into place.