Newbie to CI, don't know how to make External CSS work.

[eluser]Unknown[/eluser]
Hi all:

I'm new to CI, I don't know how to link external CSS.

Link to the file: http://www.willrain.net/CI/index.php/soho/

I've tried
<link href="<?=base_url();?>css/bootstrap-responsive.css" rel="stylesheet">
But it seems halting on base_url function.

I have

$config['base_url'] = 'http://willrain.net/CI/';

If I do this
<link href="<?php echo "aaaa" ?>css/bootstrap.css" rel="stylesheet">
It will show, but of course( not properly linked to the CSS).

So I wonder if base_url function still exists? Is there any way to solve this?

Please advice.

Thanks.