Sorry for the late reply.
Here are the steps on how to add the Buy Now button.
1. Upload to your blog the image/button you would like to use
2. Go to Plugins -> Editor and from the top right drop down menu select WpZon Plugin
3. Find the following
$seed = "See Details >>";
and replace it with
$seed = "<img src='http:// yourblog. com/wp-content/uploads/2013/08/buy-now-button.png' />";
Obviously you will need to change
http://yourblog.com/wp-content/uploads/2013/08/buy-now-button.png with the URL where you have uploaded your image.
You can add margin or padding as well.
Hope this will help you :)
Thanks for using Wpzon