I'm not sure what the problem is; running the below works as expected:
<div id="thumb0" class="thumbs" onclick="klikaj('rad1')">knock knock</div>
?<div id="rad1" style="visibility: hidden">hello world</div>????????????????????????????????
<script>
function klikaj(i) {
document.getElementById(i).style.visibility='visible';
}
</script>
See also: http://jsfiddle.net/5tD4P/