Solved by myself, this documentation could be a bit updated for easier implementation, for those who will the same problem, let me give necessary steps:

1. Install the plugin
2. Rebuild then restart NodeBB
3. Add some custom fields
4. You can edit them by going to edit profile and "Change Extra"
5. To display them in user profile, in case if you're using Person theme, go to nodebb-theme-person -> templates -> account -> profile.tpl
6. Paste the code wherever you want it do be visible

<!-- IF customFields.length -->
<div class="custom-fields">
 <!-- BEGIN customFields -->
 <div class="custom-field-item">
  <span class="field-label">{customFields.name}</span>
  <span class="field-value">{customFields.value}</span>
 </div>
 <!-- END customFields -->
</div>
<!-- ENDIF customFields.length -->

7. Rebuild then restart NodeBB