before i was only missing the loader on the back right (picture underneath)
That was entirely normal. As explained on the manual, GSX supports only two cargo loaders in total, so in the configuration for the freighter is to have the cargo loader that was originally on the back-right, to be used on the left main cargo door instead.
couldnt get this side switch from passenger doors to cargo doors in the configurator.
You don't "switch" a door in the airplane configurator, that's probably your mistake. A passenger door will always be a passenger door, and so all other doors, they can only change location, not function.
whats wrong ?
I can't possibly say what you did wrong, but the main point is you don't HAVE to configure the PMDG 747, because is a INTERNALLY SUPPORTED AIRPLANE, and the -8F version is also recognized properly.
So, to Undo any mess you might have created trying to configure something that doesn't have to be configured in the first place, use the "Reset" button on the Airplane configuration menu, to restore the configuration that comes with GSX, then "Restart Couatl".
Of course, this assumes you are ALSO properly configured the -8F model in the PMDG configuration utility, before launching FSX.