$q = db_query("select s.productID, s.name, s.Price, s.enabled, t.filename FROM ".PRODUCTS_TABLE." AS s LEFT JOIN ".PRODUCT_PICTURES."
AS t on (s.default_picture=t.photoID AND s.productID=t.productID) WHERE s.categoryID!=1 AND s.enabled=1 ORDER BY s.items_sold DESC LIMIT 0,".$cifra);
и измените на
$q = db_query("select s.productID, s.name, s.Price, s.enabled, t.filename, s.brief_description FROM ".PRODUCTS_TABLE." AS s LEFT JOIN ".PRODUCT_PICTURES."
AS t on (s.default_picture=t.photoID AND s.productID=t.productID) WHERE s.categoryID!=1 AND s.enabled=1 ORDER BY s.items_sold DESC LIMIT 0,".$cifra);
а в блоке, например в core pluserdefaultlockspopularstatic.tpl.html
для вывода описания используйте между {section}...{/section}
переменную {$popular_products.brief_description}