robots.txt in CI

[eluser]esthonwood[/eluser]
Hello, I want to implement robots.txt on my site and I want to disallow access to www.mysitename.com/index.php/admin/

Here's the content of my robots.txt:

# robots.txt for http://www.mysitename.com/
User-agent: *
Disallow: /index.php/admin/

I checked it with Google Webmaster Tools and it is still accessible. Any suggestion anyone?
Cheers!