Enter the weights in the appropriate boxes. You can navigate among the boxes with the tab key, in which case some computations are automatic.
The function of the Clear button should be obvious. The other buttons force the weight and balance computation. "Compute" will use whatever values you supply, even if it makes the airplane too heavy. The other two buttons will use as much fuel as possible, within the limit of maximum gross weight. Without allowing the airplane to exceed MGW, "Max Fuel" will use as much fuel as the airplane will hold. "Tabs Fuel" does the same thing, but will not use more fuel than the airplane holds when fueled to the tabs. This way, you can automatically figure weight and balance with the normal (to the tabs) fuel load.