Thanks for your quick and very helpful response.
Yes, I chose this plugin because it is mentioned on this page: http://www.simpleprothemes.com/add-slider-flex/ –> ‘By default, theme includes basic styling for Genesis Responsive Slider plugin.’ after which I assumed that I needed to install that plugin and then the outcome would automatically be what is described in the instruction.
From your answer I learned that I don’t need to use that plugin at all. I’ve deleted it and now the result is what I wanted.
Thank you so much!