You can use the + and - buttons to add positive and negative charges at any time, and you can always click-and-drag a charge to re-position it. The four buttons under the window will set up some standard initial configurations - make sure you click the "Field Vectors" button to look at the electric field from these configurations, and the "Contours" button will draw the equipotential lines.
Double-clicking on any point will draw the field line that passes through that point.
The last button, setting up a situation where the positive charge is 10 times larger in magnitude than the negative charge, is interesting. Add a test charge by hitting the "Test" button and move it around to see if you can find the place on the line joining the charges where the field is zero. Is it between the charges, to the left, or to the right?
This simulation is based on a physlet by Wolfgang Christian, part of the physlets page at Davidson College.