Well you are missing both the <head> and <body> sections, which most browsers regard as essential for a 'well-formed' HTML document. You should also put your Javascript code in the <head> section, unless you have a specific reason for putting it further down on the page.
<html>
<head>
<title>My Page</title>
<script language="javascript">
function LoadPage() {
var objFrame = document.getElementById("myIframe");
objFrame.src = document.getElementById("URL").value;
}
</script>
</head>
<body>
<form name="form1" method="post" action="">
<label for="urls"></label>
<input type="submit" name="back" id="back" value="<" onClick="myIframe.history.back()">
<input type="submit" name="forward" id="forward" value=">" onClick="myIframe.history.forward()">
<input type="submit" name="Search" id="Search" value="Go" onClick="LoadPage()">
<input type="text" name="urls" id="URL" value="http://">
</form>
<iframe src="http://www.google.com" width="500" height="400" frameborder="0" id="myIframe"></iframe>
</body>
</html>