Excellent!! Thanks a lot 
Looks like this now, worked for me:
Code:
$contact_name=$_SESSION['indexu_session_username'];
// mod
global $email;
$users_obj = new clsUsers;
$users_obj->table_name = "idx_users";
$email=$users_obj->GetEmailAddress($_SESSION['indexu_session_username']);
// mod
DisplayTemplate($theme_path . "add_form.html","\$error_msg,\$title,\$url,\$description,\$category,\$contact_name,\$email,\$cat,\$add_cat1,\$add_cat2,\$keywords,\$reciprocal_url,\$custom_field_form,\$sponsored_listing_period,\$premium_listing_period,\$basic_listing_price,\$currency_symbol,\$available_payment_systems"
. $custf);
}