Did you purchase DVDA from Sony or at a store? If you got it from Sony you need to register it with the key that came in the form of an email. If you bought it from the store, the key should be on the box.
Generally speaking, you get a serial key when you buy a legitimate copy of software as David mentions. Unfortunately Sony/SCS or MAGIX themselves have not sold this version any more for a while, so if you have not bought it yet you need to buy it from another reliable source.