The java visualizer will give you an outline for your class and main method. From there, all you need to do is copy/paste or write your for loop in the box beneath the main method.
Be sure to include the correct number of open and closed curly brackets. Then hit the "Visualize Execution" box under the box containing the code. Now you can see the number of steps in the loop and see what each one does with a press of the "Forward" button.
That should be all it takes to use this tool, but it will not work unless the code is typed correctly and the right amount of curly brackets are used.