If you click the left mouse button, you will set the little ball of charge in place. It will remain there until you Reset the interactive. After you put one down, another will appear on your cursor. You can place up to 10 fixed charges on the grid.

Test the electric field's effect

If you now choose the Test item in the "Add Charges" panel, you can place a "test charge" into the grid area. A "test charge" is simply a little tiny bit of charge, small enough that it doesn't have any appreciable effect on other objects. It does, however, feel the electric force due to all the fixed charges (or due to the electric field, if you prefer to think about it that way).