In the end it was very easy. An IT student friend put this:
($a['b_required'] == 1 ? '<span class="req">*</span>' : '') .
here (functions.php file):
if($linked === false) {
$html .= '<div class="control-group atr-type-' . strtolower($a['s_type']) . ' ' . ($a['s_type'] == 'CHECKBOX' ? 'atr-check-options-' . ($a['b_check_single'] == 1 ? 'single' : 'multi') : '') . '" id="atr-' . ($a['s_identifier'] <> '' ? $a['s_identifier'] : 'id' . $a['pk_i_id']) . '">';
$html .= '<label class="control-label" for="atr' . $a['pk_i_id'] . '">' . ($name <> '' ? $name : __('New attribute', 'attributes')) . ($a['b_required'] == 1 ? '<span class="req">*</span>' : '') . '</label>';
$html .= '<div class="controls">';
} else {
$html .= '<div class="atr-linked">';
}
and the problem is solved. Thanks again for the replies