Hi!
The option to enter a different URL only works if you have the correct permalink structure set up, or if you have the search page on a different url.
There are a few tutorials out there on how to do that, for example this one: http://wpengineer.com/2258/change-the-search-url-of-wordpress/
You will have to add extra functions and .htaccess rules to manage this redirection, but that tutorial works in most cases.