If its similar to the previous engine/gearbox (JTD up to 2006) there's a drain plug (square key) in the bottom the gearbox. You'll need a proper socket drive to shift it. There is no level plug on these gearboxes so you have to fill it with the correct volume of specified oil. On the JTD gearbox there is a filler/vent rubber plug on top of the gearbox which is only visible when you have removed the air filter box.
You can download the x250 workshop manual from the Fiat Ducato forum in the download section. Well worth the effort!
The 244/5 version suffered from the possibility of rain water draining from the windscreen scuttle via a drain pipe directly onto the gearbox and water actually getting into the gearbox!
Don't know if they have resolved this on the x250. Seems all Fiat Ducatos come with built in ornamental water features designed to keep their owners on their toes.