It should be at the very top! I couldn't get your main url to load, it finally took me to the forum though so can't see this to tell.
Example:
Code:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
Code blah blah blah
</head>
<body>
Code blah blah blah
</body>
</html>
This would be at the very top, the <html> is directly below it and after the closing tag for body i.e. </body> at the bottom you must close the html i.e. </html> then you're set

.
This should only be you trying your hand at html/css, you should not be removing or adding this code from vBulletin or it's templates unless you're experienced doing so as it could cause display and/or functionality issues if not done correctly.