I think You are using //--style="display:none"--//
for hiding the div.
Use this code:
<script>
function klikaj(i) {
document.getElementById(i).style.display = 'block';
}
</script>
<div id="thumb0" class="thumbs" onclick="klikaj('rad1')">Click Me..!</div>
<div id="rad1" class="thumbs" style="display:none">Helloooooo</div>