well you need simply to put 1 kg, in the alcove that ask it, the east alcove if I remember correctly. Then 2kg in the next alcove (multiply it by 2) starting on the right, then 4kg, then 8kg.
it is better if you put items in a sack before putting them in the alcove, if you don't have (an item with the exact weight). Some player did have problem with that.
Did you find your missing key?
I think I have made a confusion with the room you talk, can you post a screenshot of your map please? I don't know which round key you need, as in this level there are 2 needed.
Anyway if that is the big room with the scavenger and the skeletons, (that you already opened with a round key), you now need another round key to go to the end of the level, this one is put in an hidden area, somewhere in the big room or near, you need to find 2 tiny buttons to open this area. The round key is somewhere in this area.
No problem for the help, well I don't notice that any nodders did not help the players

this is quite normal.