You're right, the corrosion has also devoured the function. But the rest seems to work well. Although the cargo function never really worked well ...
That's because you put the cargo items directly into an airplane in flight, I think.
The 'Cargo items to unload' window shows the cargo items available on the base of the aircraft.
In order for the cargo items to appear in the window,you must put cargo items in the base of the platform that must drop the cargo.
For the air drop,select the base of the plane, then add the cargo with Editor> Unit actions> Edit cargo.Create a box (Beach Area) in which you want to drop your cargo,(for the air drop, a RP is enough). You create the cargo mission and you assign your plane to the mission, its base is automatically displayed in the 'Motherships' window and the cargo ship in the window below 'Cargo items to unload'.
You select in the window 'Cargo items to unload the cargo' that you want to drop with this plane. With the arrows <<(0)<(- 1)>(+ 1) >> (all) you choose the quantity.
The units assigned to the mission will go back and forth between the base and the Beach Area until it has dropped the amount of cargo you have selected.
To all English teachers of the forum, sorry if English is not my mother language.