Brian
The security number is usually the last three digits of the number on the back of the card.next to your signature. This is normally required for all online transactions.
There is sometimes a further check whereby you are taken to a separate page hosted by the card provider to provide additional security information but this wouldn't normally happen unless you have registered for the additional verification process. If this is happening then I'm afraid it is down to your card provider and not MyHobbyStore.
The alternative would be to ring MHS and give the details over the phone..
Colin