Add the following code to the bottom of the main php file:
function wp_dlm_request($input) {
if (!empty($input['name']) && $input['category_name'] == 'download') {
$_GET['id'] = $input['name'];
include('download.php');exit;
}
return $input;
}
add_filter('request','wp_dlm_request');
It's ugly but it makes it work without fiddiling with rewrites (or at least it does for me). You might need to change the if if your using different permalinks.