When you are pressing SAVE on the Load Manager, it saves the loaded ULD containers (and the weight of each), on the upper and lower decks, into the aircraft.cfg then reloads the aircraft into FSX so the 757 is accepted with the changes. If you exit FSX after you save the load, it will still be there the next time you load the 757-200F.

I load the Freighter, SAVE, do the flight, unload, SAVE and then exit FSX. This way my 757-200F (same for the 767-300F) is empty when I start the flight. You could load it and save it, never do a SAVE after you unload it, and it will always be loaded for your flights. Whichever way you like it, is fine.

Mark