That code may have expired. After doing a little looking around, I noticed some recent reviews on amazon.com that stated the codes for that rifle expired in 2010.
If I were you, I'd send off a ticket to support and ask them to verify the expiration of the code. If it's expired, you're pretty much out of luck.
I don't think codes like that actually expire for MMOs. Might be different for STO though.
It sounds like the OP had applied it to his account. If it was expired it shouldn't have allowed him to apply the code in the first place.
I would recommend contacting support again, giving them the serial key in the support ticket, and ask them to make sure the code was valid, and if so, whether or not it actually applied to your account.
If the code turns out to not be valid or expired you would need to contact the person you bought it from to see about getting a refund, but I doubt that would happen because they might think you are lying about it not being valid after successfully using it when you are telling the truth. So unfortunately if the code wasn't valid or expired you would be out of luck. Hopefully you didn't pay too much for it if that were the case.
If you can't get any help from support, I'd suggest sending pwebranflakes a private message through the forums. He is the community manager and might be able to get them off their lazy bums and look into the matter.
Hail your new Overlord!
Last edited by monkeybone13; 09-11-2013 at 06:55 PM.