Hello Falk,
The login and register positions show inside the S5 box. The positions are inside this module not on the page this is why they aren't showing. You would need to add a custom module position. Here is some code to help you add one. This is "custom_6" so once you add the below to wherever you desire in your index.php file:
<?php if($s5_pos_custom_6 == "published" && $s5_menu_position_type == "flex") { ?>
<div id="s5_pos_custom_6">
<?php s5_module_call('custom_6','notitle'); ?>
</div>
<?php } ?>