FSX can't find the MDL file (probably the 707-300 VC) that the 707-300C requires.

First and foremost: The 707-300 base pack required - The 707-300C won't work without it.

1) Do you have the 707-300 Base Pack? NO? You need to buy it. YES? See #2.
2) Is it installed? NO? You need to install it. YES? Then you need to uninstall then reinstall the 707-300C Expansion Model.

Mark