you missing the link to the /SpryAssets/SpryValidationCheckbox.js file
change:
<link href="/SpryAssets/SpryValidationCheckbox.css" rel="stylesheet" type="text/css" />
to:
<link href="/SpryAssets/SpryValidationCheckbox.css" rel="stylesheet" type="text/css" />
<script src="/SpryAssets/SpryValidationCheckbox.js" type="text/javascript"></script>