You need to add the css and the javascript libraries for semantic-ui. Here try this example:
<html>
<head>
<link rel="stylesheet" href="//cdnjs.cloudflare.com/ajax/libs/semantic-ui/0.12.0/css/semantic.min.css"/>
<script src="http://cdnjs.cloudflare.com/ajax/libs/semantic-ui/0.12.0/javascript/semantic.min.js"></script>
</head>
<body>
<div class="ui labeled icon button">
Download <i class="download icon"></i>
</div>
<div class="ui icon button">
<i class="download icon"></i>
</div>
<div class="ui button">
Download
</div>
<div class="ui facebook button">
<i class="facebook icon"></i>
Facebook
</div>
</body>
</html>
Here is a jsbin POC: http://jsbin.com/ubiReqa/1/
Hope it helps.
Cheers, Praney