In order to edit your reseller site from your Eco Web Hosting account go to
Reseller Services: Your Greenhost Shop > Edit Greenhost Site
From this page you will be able to be able to edit the various files/pages that form your Greenhost site. Each page is built with the universal header and footer so for changes that will need to apply to all pages (eg. adding a link to a CSS file) you can make your changes to these two files.
For pages such as hosting or domains you will need to add code to display your products on the corresponding files, creating and setting hosting/domains to be sold through the site will not add these products to this page automatically.