I think you have missed below file..
<script src="http://code.jquery.com/jquery-1.4.2.js " type="text/javascript"></script>
Include this and then check...
If you don't have internet connected, download the file and place it in project folder and then link properly..
<head>
<title>Div adding</title>
<script src="http://code.jquery.com/jquery-1.4.2.js " type="text/javascript"></script>
<script type="text/javascript">
$(document).ready(function() {
$("#aBAddClnBtn").click(function() {
var new_div = "<div style='border: solid 1px gray; width: 15%; '><div style='border: solid 1px gray; height: 50px;'> </div> <div style='border: solid 1px gray; height: 445px;'> </div></div>";
$(new_div).css("float", "left", "margin", "10px").appendTo("#divContent");
return false;
});
});
</script>
</head>
<body>
<div id="divContent" style="border: solid 3px gray; height: 500px; width: 100%;">
</div>
<input type="button" id="aBAddClnBtn" value="Add Column" onclick="" />
</body>
</html>