Admin CP and Sub-Directory

You dont need any routes for this, CI will work out of the box. CI automaticly detects a folder like 'admin' with one or more controllerfiles (like Login.php) in it and adds a internal route for site.com/admin/login. Atleast At the top of my head I cant recall modifing the routes manual for this.

Please mind that the tutorial is from 2009(!), a few things have chenged since then, one for example wich could explain your issue is that controller files now should start with a capital character, try renameing your file to admin/Dashboard.php and check the upgrade documentation for other changes that might effect you if you try examples from so long ago.