Assuming you have purchased the software direct from Magix your serial number(s) and download links will be in the confirmation email you will have received, if you do not have this check your spam folders.
If purchased from a retailer you may have received a coupon/voucher code - in this case you need to redeem the voucher here and you will then be sent the serial number(s) by email.
If you are trying the trial version, then you have the option to extend the trial to 30 days, once you have registered to exten the trial period, then you will be sent the trial activation code.