Hi,
Basically, it is not really possible like that to display the category name in the product page.
The reason is simple : a product can have multiple categories. You can't know in which one you are except if you use a category list (like the breadcrumb do).
But in the product page view, you can access to " $this->categories " which is an array of the product categories.
It contains the categories object, so you can do
$first_category = reset($this->categories);
echo $first_category->category_name;
If you want to display the category name of the first category of the product.
But be careful, like I said in the beginning of the message, you can have wrong display if your product have several categories and the navigation can display the product from another category than the first product category.
Regards,