Unzip the element and put it in your project's Common/Elements folder
Open your project
Insert a group of elements where you want on your page. Consider that this element will disapear on window opening. The footer might be a good place to put it.
Add your message on this group of elements using all standard techniques.
Insert the Modal Window Element into your page.
Select the group of element containing your message.
Select the Close pannel cross color
Select frequency of display.
If you selected Day or Once and you decide to change the displayed message, make sure to change the version number.
Select background color and opacity.
Use OE's Edit Media or media queries to make it responsive.