Hello,
In the settings of the Joomla user manager, you want to activate the creation of new user accounts. That will activate the registration form interface so that you can register, and after you do, you'll see the finish button there.
The Nicolas message is talking about the Joomla part of your backend ; not the HikaShop one.
I'm afraid that Joomla related question are outside the limit of our support but you can find a lot of documentation in the Joomla website and find some help in the Joomla forum.
Thank you for your understanding.