Unfortunately, your first post is considerably lacking in detail so I have to guess what you are asking!
If you have already bought the software from Magix, the serial number will be included in the email you received confirming the purchase.
If you have bought the software from a third-party seller, it should include something like a voucher code which you need to redeem here. Full instructions can be found here.
If you have previously registered and activated the software but need the serial number to reactivate it go here, log in, go to My products and find the listing for your software. The serial number will be there.
If you have purchased a used (second-hand) copy of the software you need to ask the seller for the serial number and ensure that he/she has told Magix that they have sold the software and to remove their registration details. Failure to do this will mean you will never be able to register and activate the software in your own name!
I can't think of any more possible scenarios so I hope this helps!
I removed the 1 registration on file, but the program download you have there as my download - once installed, upon first run attempt and after, will not register/ activate online - "Need internet connection" - obviously I have internet (writing here) - even removed firewall and virus protections but still same message - ("Need internet connection" box - with button options to "reset internet connections", which I tried, and "retry" or "quit.") So.... I also noticed other bookmarked links I had for this program "loop" or redirect back to login... ?????