you actually don't need usgn to play. click "find servers" and join. if you don't see servers: check your firewall settings or check the steps listed at the official FAQ page:
www.cs2d.com/faq
make sure that your usgn login name and password are correct and that your account is a valid and activated usgn.de account (you can't login with unrealsoftware.de accounts in games!)
also check the console for error messages