Check this out. I think you put this widget below a structure like in that link. So, you should have a div (container) with the class "flexslider", then a container with the class "slides", then a container with the content of each slide inside that. The widget converts that structure into a slider. If you do that, you can use the default settings in the widget's "Setup" tab.
I didn't know about the flexslider widget until now! You might also consider looking at the FormCarousel widget I just created. It allows you to define a list of Mendix pages/forms to load inside your slider/carousel. I think that's more intuitive than the flexslider approach for many uses - I'll have to try out FlexSlider
EDIT: also see the BootstrapCarousel widget, which I used as the base for FormCarousel. It's designed to show a carousel of images.