When you applied the software the first time and got that activation code after giving your serial from Sony , you now have to make a Magix account with the same email addres and name. You than can go to that account and look for "my products" to regain the serial on that site at this you can also download the latest build following the download link on that place.
I tested this once again now by logging in there with my old SCS account email address and password. The advice in point 6 in that FAQ is still valid. You may need to have bought the Sony/SCS version of MSP 13 though, not the Magix rebrand. (Pressing download there leads to the Magix site.)