In order to help you we need you to answer some questions:
Which version of SFAS did you buy?
Where, exactly, did you buy it from? If it was direct from Magix, the confirmation email you received should include the missing serial key. Have you checked it right to the end (these numbers are often found towards the end of such emails)? If you bought it from a third-party seller you should have received a voucher code to redeem your purchase; that would have generated a similar email.
If you are certain that any email you received does not contain the serial key you need you will have to email infoservice@magix.net, giving FULL details of your purchase (order number etc.) and request their help in finding the key.