Hi,
There is no next button before the payment and shipping selection interface.
The next button in the HikaShop checkout is always at the end of the page when displayed.
Before registration, there is a "register" button which is displayed at the end of the registration form.
Looking at your checkout it seems that you renamed it "next".
If you want to add the next button at the end of the page, you can remove the line:
$this->nextButton = false;
from the file "login" of the view "checkout" via the menu Display>Views.