I've purchased your plugin and the backend of it all looks great.
I've run in to some problems though. When adding the shortcode of a post form to a page, it does not show the form. I've debugged it down to the fact that the following variable $form_vars = wpuf_get_form_fields( $form_id ); in class/render-form.php on line 263 returns an empty array. In the backend all the fields show as expected.
This can happen for plugin and theme conflict. Disable all your plugins except User Frontend and activate a default theme of WordPress like – twenty fourteen and see if the issue remains or not.
And please never ever change our plugin files. Make all the changes in your theme.