When you drag your Reflection Stack out, you will see the above stack appear on your page.
Now you can drag your desired image into the image drop area, or if you want every image in a group of images to have the relfection effect, chagne the Apply FX setting from Single Image to All Images Added to Stack.
Set your desired settings in the settings panel (top radius, padding, opacity & height).
Now preview and enjoy! For an overview of the remaining settings please refer to the Reflection Settings section.