Are you trying to get Zen Cart functions dropped into spots in your HTML pages? That is not going to work. ZC is designed to function as a complete site, controlling its own pages.

It is possible to install Zen Cart in a subfolder, style it to look like your main site, and have links to go from one to the other. This does run the risk of losing the session and customers' cart contents if they leave the ZC part for a long time.

Have you tried styling the Zen Cart home page (define_main_page.php in Tools > Define Pages Editor), or the product listing page or product info page?