Assuming you have already installed the software and registered it?
Go to the Magix website, top right of the home page you will see My Account - click this and login with your email address and password you used to set up your Magix account.
Select My Products on the next page - here you will find a list of your registered products and the serial numbers and download options if available.
If you have not registered the software, if you bought the software from Magix as a download - you need to contact Sales at Magix