Hey there,
I am really sorry for the delay of this reply as it is a weekend for us here.
You can modify on this file ->
JoomlaFolder\components\com_easydiscuss\themes\wireframe\toolbar\default.php
// LINE 320
// Change this `ED::getRegistrationLink()` to your registration page.
<div class="popbox-dropdown__meta"><?php echo JText::sprintf('COM_EASYDISCUSS_TOOLBAR_NEW_USERS_REGISTRATION', ED::getRegistrationLink());?></div>
Hope this help.