address for include files

Problem
My include files cannot be seen by the browser.

Scenario
I’ve added a folder with some files that I want to include on a page in my blog.

The set up is a follows
root folder (it’s a subdomain)
added folder (-assets)
included files folder
file to include(formButton.html)
wp-content
themes
twentyten folder
header.php

Inside the header folder I’ve added the following:

<?php 

 	include ('../../_assets/include_files/formButton.html');

?>

It appears to me that the route to the file for the include statement is two folders up but the browser can’t find the file.

Any suggestions on the cause of the problem would be greatly appreciated.

Thanks