Hi Paco,
You can open the weight and fuel by going to the Aircraft menu -> Weight and fuel. Drag the sliders with your mouse to change the settings. If you want to change it mid flight, I would recommend using the equipment failures also found under the aircraft menu.
The command "operation/slider #" will only work if a slider has been matched to that number in Plane Maker. The 777 is from X-plane 9 or earlier, so it may not have all the features available in X-Plane 10. What were you hoping to accomplish by using slider 11?