William,
Not sure if this is relevant to all boxes, but have heard that the best way to fill the 2.0 gearbox is through the reversing light switch on the top of the box.
On the Boxer version it has a filler/level plug at the rear of the box just above the speedo sensor. I would advise caution when filling the box as if the fiat version does not have a level plug there could be a danger of overfilling. The safest way would be to drain totally and refull the correct amount in via the reverse light switch. Not sure how much your box holds, dealer should be able to advise.
Regards
Stewart