I cleaned up your code, and put all the header information for installing it. Your plugin works fine, I tested it!
/*
Author: whoever
Plugin Name: Some Name
Plugin URI:
Text Domain: some-domain
Version: 1.0
*/
function DynamiteSearch($form)
{
$form = "<form method = 'get' id = 'searchform' action = ' ".site_url()." ' >";
$form .= "<div><label class="hidden" for="s">". __("Search for: ") . "</label>";
$form .= "<input type="text" value=" ". attribute_escape(apply_filters("the_search_query', get_search_query())) ." ' name="s" id='s' />";
$form .= "<input type="submit" id='searchsubmit' value=" ".attribute_escape(__("Look for it'))." ' />";
$form .= "</div>";
$form .= "</form>";
return $form;
}
add_filter('get_search_form', 'DynamiteSearch');
Try changing the submit button text to see if it works. Let me know if it worked for you.
Thanks,
Rutwick