Can you post the contents of your .htaccess, a screen shot of your current settings for "SEO URLs", a list of plugin / modules installed, and a link to your site please? Also do you see this behavior when using the template / theme Zen Cart comes with?

I am especially interested in the following lines from your .htaccess.
Code:
RewriteBase /shop/

# Normal operation Ultimate SEO URLs
RewriteRule ^(.*)-c-([^/])/(.*)-p-(.*).html$ index\.php?main_page=product_info&products_id=$4&cpath=$2&%{QUERY_STRING} [L]
RewriteRule ^(.*)-p-(.*).html$ index\.php?main_page=product_info&products_id=$2&%{QUERY_STRING} [L]
Those two lines are vital as they ensure "action=add_product" is sent to Zen Cart. The order the rules are listed in the .htaccess file are also important.