Once you set up your products in E-junkie Seller Admin, we provide ready-made HTML codes for your cart buttons. You would simply copy the codes from us and paste them into the HTML source of your own Web site pages, wherever you want our buttons to appear among your own layout, text and images. Our standard cart would appear as an overlay "inside" your own page, fitting within your site's existing layout and branding, or you can use Buy Now buttons that bypass the cart to take buyers directly to instant checkout for just one item at a time.
This help page provides further tips you may find useful, including a section for WordPress in particular:
http://www.e-junkie.com/ej/help.site-blog.htm