You can open the answer.php file in the templates folder of dw-question-answer then add the following code to the line 26:
<?php elseif ( ! is_user_logged_in() ) : ?>
<div class="alert"><?php _e( 'You do not have permission to submit a question','dwqa' ) ?></div>
<div class="dwqa-answers-login">
<div class="dwqa-answers-login-title">
<p><?php printf( __( 'Please login or %1$sRegister%2$s to submit your answer', 'dwqa' ), '<a href="'.$link_register.'">', '</a>' ) ?></p>
</div>
<div class="dwqa-answers-login-content">
<?php wp_login_form(); ?>
<?php do_action( 'wordpress_social_login' ); ?>
</div>
</div>
See the screenshot: http://prntscr.com/inwkhf
