are we talking about the single product view or the product archive pages (shop,categories)?
because the text beside the checkbox/option states, roughly translated:
»activate ajax-checkout-button on product archive pages«
and on all the installations i did so for, that is the way its working – ajax checkout on the archives, but not on the single view.
the latter you have to implement yourself or maybe you find a free plugin for that.
Follow up:
A basic example on how to use AJAX for the »add-to-cart«-functionality of woocommerce can be found here:
Woocommerce – Add a product to cart programmatically via JS or PHP.