Ok, the problem is, that Kunena doesn’t support 3rd party mods for user profiles. As I know the only way to add fields to Kunena profile is the following:
In the file:
components/com_kunena/funcs/profile.php
you have to find the line (line 205)
$form = JForm::getInstance('com_users.profile', 'frontend');
and replace it with the followings:
JPluginHelper::importPlugin('user');
$form = JForm::getInstance('com_users.profile','frontend');
$dispatcher = JDispatcher::getInstance();
$dispatcher->trigger('onContentPrepareForm', array($form, $data));
With this modification Kunena will load Joomla user profile plugins.