You have to assign an id to the table, then append to the tbody of this table via the id, see example:
<!DOCTYPE html>
<html>
<head>
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.11.2/jquery.min.js"></script>
<script>
$(document).ready(function(){
$("#btnAdd").click(function(){
$("#mytable tbody").append("<tr><td>col1</td><td>col2</td></tr>");
});
});
</script>
</head>
<body>
<table id="mytable" border="1">
<thead>
<tr>
<th>Header 1</th><th>Header 2</th>
</tr>
</thead>
<tbody>
</tbody>
</table>
<button id="btnAdd">Add Row...</button>
</body>
</html>