Assuming you use the PMDG 777F, it seems the configuration is correct, because the door is named Main Cargo Door (well, not exactly, is supposed to be named "Main Deck Cargo", is it ? If not, you HAVE a problem with the configuration ).
If the configuration is correct, the only possible reason why you don't see any loader, is that you haven't installed the vehicle, which is not possible, since it's included BOTH in the 1.8 Full Installer AND in the vehicles update (there wasn't any need to install the vehicles update, if you used the 1.8 Full installer)
Do you have an FSDT_Cargo_Loader_HI folder in the FSX\Simobjects\Misc folder ? You should, since it gets installed by either the 1.8 Full installer or by every vehicles update installer.
If not, reinstall using the Full installer only. No need to install the vehicles updates, if you use the 1.8 Full installer.
Of course, I've tried your exact same situation, at KLAS parking 901, and it works without any problem, see the screenshot.