I have the FS 2004 and FSX "Air America" C123 craft.
Have a peek at the "readme file" enclosed with the aircraft for instructions.
I nominated Shift+T to open the rear cargo Doors, and "/" to remove/replace the cargo when on the ground.
There will be other defined key presses to carry out the other options; ladder, refuel, etc....
Paul....
...!
Win 8.1 64-bit. DX11. Advent Tower. Intel i7-3770 3.9 GHz 8-core. 8 GB System RAM. AMD Radeon HD 7700 1GB RAM. DVD ROM. 2 Terra Byte SATA Hard Drive. Philips 17" LCD Monitor. Saitek Cyborg X Fly-5 Joystick. ...and a Briggs and Stratton Petrol Lawn Mower.