How can I show a hidden div when a select option is selected?

Try handling the change event of the select and using this.value to determine whether it's 'Yes' or not.

jsFiddle

JS

document.getElementById('test').addEventListener('change', function () {
    var style = this.value == 1 ? 'block' : 'none';
    document.getElementById('hidden_div').style.display = style;
});

HTML

<select id="test" name="form_select">
   <option value="0">No</option>
   <option value ="1">Yes</option>
</select>

<div id="hidden_div" style="display: none;">Hello hidden content</div>

try this:

function showDiv(divId, element)
{
    document.getElementById(divId).style.display = element.value == 1 ? 'block' : 'none';
}

#hidden_div {
    display: none;
}

<select id="test" name="form_select" onchange="showDiv('hidden_div', this)">
   <option value="0">No</option>
   <option value="1">Yes</option>
</select>
<div id="hidden_div">This is a hidden div</div>