Maligna, I'm going to presume that you used a RoM-branded game card to buy diamonds. If this is the case, please check the Junk/Spam folder for the email address registered with your game account. Even though you received a voucher code for the first card you used, some mail providers have a tendency to tag email from Frogster as Junk or Spam mail, and the voucher email may have landed there.
If you used the card with a second game account, you will have to check the email address that is registered with the second account.
If you really didn't receive a mount voucher code, and since this is technically a billing issue, you will have to contact Support. You can use the link in my sig below or one of the Support links on the main page of the website. If you've not contacted Support through our Support Portal in the past, you'll have to create a Support account with them. Explain your issue as clearly and completely as possible, and be sure to include the following information in the ticket:
Game account name
Server played on
Main character name.
It may take a few days for Support to send you a non-automated response. Follow whatever instructions the Support personnel give you. And remember what I said above--check your Junk/Spam mail folder, along with your Inbox.