If you cannot find your voucher, it may be because it has already expired, or because you have it in a different Live Your City account.
To ensure it is not the latter case, please make sure that you've logged into the website using the same login method you used to make the purchase (email, Facebook, Google).
You can check the expiration date of your voucher (and any other conditions it has) by tapping on it. Vouchers disappear once they expire, so make sure to check the expiration date ahead of time!