In order to enable registration through JoomSuite User component, you need to hack the Joomla Login module and change the link from com_user to com_juser.
To hack Joomla Login module:
1. Open the file "joomla root/modules/mod_login/tmpl/default.php" in a text editor, such as Notepad.
2. Find the following lines:
<a href="<?php echo JRoute::_( 'index.php?option=com_user&task=register' ); ?>">
<?php echo JText::_('REGISTER'); ?>
3. Replace "com_user" with "com_juser" to enable registration through JoomSuite User.
4. Save the file.
At times, registration through JoomSuite User is not enabled even after performing the above mentioned steps. This happens when Joomla 1.5 templates have custom mod_login template. In such cases, open the custom template located at "/templates/tpl_name/html/mod_login/" and perform step 2 to step 4.