Routes are defined as such:
$route['product/:num'] = "catalog/product_lookup";
where product
/ anynumber
is your URL: http://www.example.com/product/234. The second part is the controller/method. The URL we're talking about here would call the catalog controller's product_lookup method.
$route['accounts/mainaccount'] = "adminController/mainAccount"
Would fire what you're looking for. Take a look at the CodeIgniter User Guide for more examples.
I use this .htaccess
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /index.php?/$1 [L]
Which allows any physically existing file to be accessed, all else is routed to CI.