Which version of NS are you talking about? This works in 6.2:

Code:
<form id="form1" action="#">
  <select id="select1">
    <option>Option 1</option>
    <option>Option 2</option>
    <option>Option 3</option>
    <option>Option 4</option>
    <option>Option 5</option>
  </select>
  <input type="button" value="Select"
  onclick="document.forms['form1'].select1.options[3].selected = true;" />
</form>