To add new pages, you must first provide write permissions to your theme folder (CHMOD 777) and all folders and files within it or specifically to the /themes/YOURTHEME/ folder. Start by going to the "Add New Templates" link & click the "ADD" button/icon, once you have entered your template name. You should now be able to select your new page from the templates dropdown. You can immediately begin adding your content/html.
Note: If adding content pages, add the prefix "page_" to your template names (See image.) and you can link to your pages from anywhere within the shop.
(Example: <a href="index.php?l=page_view&p=new_template" target="_parent">Link to New Page</a>)
We also have a detailed tutorial on how to add additional pages within Sunshop at our Knowledgebase: http://my.twt-inc.com/knowledgebase/35/Adding-additional-pages-within-SunShop.html