prova questo
codice:
<script language="javascript">
function viewText(valore){
	if (valore=="seconda"){
		campotesto.style.display=""  
	}else{
		campotesto.style.display="none"  
	}
	
}
</script>
<select name="pluto" onchange="viewText(pluto.options[selectedIndex].id);">
	<option id=""></option>
	<option id="prima">prima</option>
	<option id="seconda">seconda</option>
	<option id="terza">seconda</option>
</select>


<input type="text" name="campotesto" value="" style="display:none">
ciao