The objective of Mass Attack is to drop weights on the scales to balance them out.
Drop Weight: Hold left mouse button to increase size, release to drop.
Practice to get an idea of how different sizes affect the scales. In later levels, you will have less room for error, so be as precise as you can. If there are multiple weights on one platform, it may be difficult to tell how much they weigh all together. Watch how the scales change when you drop the first weight to get a better idea.