The SAVE function in ACE for the freighter only saves the ULD weights for the upper deck (large) and lower deck (small) containers. It is explained in 757 Captain FLIGHT MANUAL Part I – User’s Manual (cs757_manual1.pdf), page 26.

It has this effect - If I save them as 2000 lbs (large), then every time I load a container on the upper deck I will add 2000 lbs of weight to the 757. Every time I load a small container on the lower deck I will add 1000 lbs of weight to the 757. When you are finished with the load mangers in FSX on the 757, you must save click on 'SAVE' after you have loaded it or 757's payload will remain empty. I load the 757 upper and lower decks at the same time. I click save on ONE of the load managers (I don't need to save with both) to save the Cargo load. Once you have clicked on save on the payload managers, the 757 will retain the load untill you save after unloading!

Mark