<INPUT name="checkboxN"  id="checkboxN"  value="1"  type="checkbox" onclick="dis()">
<script>
function dis()
{
var obj=document.getElementById("checkboxN");
if (obj.checked) document.getElementById("saveSearchIdd").style.display="block";
else document.getElementById("saveSearchIdd").style.display="none";
}
</script>

------解决方案--------------------
JScript code

<script language="javascript" type="text/javascript" src="Scripts/jquery-1.4.2.js"></script>
<script language="javascript" type="text/javascript">
    $(function () {
        $("#checkboxN").click(function () {
            if ($("#checkboxN").is(":checked")) {
                $("#saveSearchIdd").css("display", "block");
            }
            else {
                $("#saveSearchIdd").css("display", "none");
            }
        });
    });
</script>