Use the Support page or the support email. Put the order ID in the first line. Add the plan length, your Windows version, and a short description of what failed. If there is an error popup, quote the text or attach a shot of it. Say what you already tried, such as running the loader as admin or reinstalling after a patch.
Questions that are already answered will get a faster reply if you mention you read Setup, Updates, or the refund policy. Do not send your card number or your game password. We only need the order details and the problem. Keep the email thread so the next reply stays tied to the same order.