I have no idea where you got that code from, use this
<?php wp_dropdown_categories('show_option_none=Select category'); ?>
<script type="text/javascript"><!--
var dropdown = document.getElementById("cat");
function onCatChange() {
if ( dropdown.options[dropdown.selectedIndex].value > 0 ) {
location.href = "<?php echo get_option('home');
?>/?cat="+dropdown.options[dropdown.selectedIndex].value;
}
}
dropdown.onchange = onCatChange;
--></script>