@RICHARD-WINEMILLER Does your code start with P3? If not, it may be a voucher code that you need to enter here http://www.magix.com/redeem/ If you enter a P3 serial number, don't forget to enter the hyphens if they are not filled in automatically.
What exactly did you buy? The only boxed things I can see on the website is the double pack of Sound Forge Audio Studio and Acid Music Studio (except the box of Music Maker and Samplitude Music Studio).
@RICHARD-WINEMILLER If the code is already used and you didn't use it in the past (in this case you should have gotten a serial number to your account and e-mail) then you need to get in contact with Best Buy to get a replacement or a refund. A code can only be used once. You will not get it to work.
Maybe somebody returned the software to Best Buy and they just resold it to you, not knowing that it's now useless.
Was the package sealed when you bought it? If not, it was probably opened by someone else who used the code before you.