I have exactly the same issue as many others have highlighted. Signed up to BT Fibre 2 on 23/03, activated 08/04. I had the email telling me to log into MyBT to claim. When you log in there is no obvious place to claim. We are really pleased with the service from the broadband having had issues with Plusnet before. The reward card was the incentive to come to BT so we feel a little let down by what has otherwise been a good start on BT.
I'm sorry for the problems you're having claiming for your reward card. I can understand you feel let down as the reward card was the incentive to move over - I would feel the same. I'm really pleased to see your broadband is performing well.
I have sent you a private message with instructions on how you can send us over your details and we'll look into this for you. See: Private messages