Make sure the anti-reversal latch is sitting against the Bevel Gear (the lowest one in the gearbox) and NOT the Spur grear (the largest, middle one).
When you put the bevel gear in you need to hold the latch “open” so that when you put in the bevel gear the spring tension closes the latch onto the smooth large notches on the bevel gear.
If you are uncertain of if position, just google about pictures such as here:
http://s144.photobucket.com/user/wol...ls009.jpg.html