Our Getting Started help page is a good place to begin setting up with E-junkie:
http://www.e-junkie.com/ej/help.sell.htm
Once you set up a product in your E-junkie Seller Admin (and uploading a file to offer as the product download is one step of that procedure), we would provide ready-made HTML codes that you just copy from us and paste into the HTML source of your own Web site pages, wherever you want our buttons to appear among your own layout, text and images.
The actual method for pasting our HTML code into your pages would vary according to what software you use to design your Web pages, so you would need to contact the software for specific instructions on that, but in general you should look for a button or menu item that lets you edit the raw HTML source directly or lets you paste a snippet of raw HTML into your page.