sebaiknya pake jquery agar lebih mudah...
<html>
<head>
</head>
<body>
<script src="//code.jquery.com/jquery-1.10.2.js"></script>
<div id="body">
<form id="login" method="post" action="">
<div>
<h1><center><b>Laporan Pegawai</b></center>
<br><br><b>Cari Berdasarkan</b> :
<select id="form" name="pencarian">
<option value="1">Laporan Golongan
<option value="2"> Laporan Promosi
</select>
</h1>
</div>
<div id="select1" class="hasil">
<form id="Form2" name="Form2" method="post" action="form3.php">
Unit Kerja :<select id="form" name="unit" >
<option value="tanaman">tanaman</option>
<option value="pengelolahan"> pengolahan</option>
</select>
</form>
</div>
<div id="select2" class="hasil">
<form id="Form2" name="Form2" method="post" action="form3.php">
Nama pegawai atau NIP :<input name="cari" type="text" id="form" placeholder="NIP atau Nama Pegawai" size="40">
</form>
</div>
<div id="tombol" >
<input type="submit" name="submit" id="submit" value="cari">
</div>
</div>
</form>
<script>
$(document).ready(function(){
$('.hasil').hide();
$('#tombol').hide();
$('#form').change(function(){
if($('#form').val()=='1'){
$('#tombol').show();
$('#select1').show();
$('#select2').hide();
}else if($('#form').val()=='2'){
$('#tombol').show();
$('#select2').show();
$('#select1').hide();
}
});
});
</script>
</div>
</body>
</head>