How do I get mouse response for a 4*4 dot matrix pattern?

I am trying to create a visuospatial short term memory task where the participant will be presented with a pattern of dots in a 4*4 matrix for about 1 second which they have to memorize. After a gap of about 2 seconds, the participant will be presented with a blank grid into which they should click with the mouse to reproduce the same pattern. I would also want the participant to be able to remove a dot by clicking again.

So here is my problem: I have been able to present a 4*4 grid with a randomized pattern of dot stimuli in it which goes about in a loop. I have done this without writing any code i.e. I have made a grid image in the background and added 16 polygons on the builder window. In the conditions file, I have specified the visibility of each dot (0 = invisible, 1 = visible). That is, I have 16 values on each row to control the visibility of the dots on that particular trial.
Next, I have gone back to each of my dot stimulus components and entered the appropriate column name in the opacity field of each stimulus (i.e. dot_1, dot_2, etc).

But how do I get the response from the participant?