It could be a redemption code and not a serial number. Check that first. Redemption Code Help
If they will not resolve the problem or talk to you, you can file a dispute with your card company or bank, and change that card number.
You have 30 days.
You could have bought direct from Adobe. A full license is $700. Anyone selling it for less is suspect.
If you are going to school, Photoshop CS6 Extended is $350 Creative Suite 6