Your script already selects the treediv element . Thus, there is no need to place the script tag inside the div tag.
<!doctype html>
<html lang="en">
<head>
<link rel="stylesheet" href="app/listTree/treenode.css">
</head>
<body>
<h3>It <code>TreeNodes</code> all the way down</h3>
<p>Edit the <code>tree</code> variable to change the tree</p>
<div id="tree">
</div>
<script src="listTree/TreeNode.js" type="text/jsx"></script>
</body>
</html>
You also lack tags <head>and <body>html.
In addition, if you want to display jsx in your React code, you also need to add tags <script>(and type="text/jsx"as a tag attribute):
<!doctype html>
<html lang="en">
<head>
<link rel="stylesheet" href="app/listTree/treenode.css">
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/0.13.3/react.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/0.13.3/JSXTransformer.js"></script>
</head>
<body>
<h3>It <code>TreeNodes</code> all the way down</h3>
<p>Edit the <code>tree</code> variable to change the tree</p>
<div id="tree">
</div>
<script src="listTree/TreeNode.js" type="text/jsx"></script>
</body>
</html>
EDIT:
, , ( . Treenode.js :
React.render(
<div>Testing...</div>,
document.getElementById("tree")
);
. Testing... rendered, , .