If so then MMM15 is not compatible with Windows 10 and AFAIK the old soundpools etc are no longer available, and you need to update to a Windows 10 compatible version.
If you mean Music Maker 2015 then you can get your serial number by browsing to magix.com and clicking the My Account icon top right of the home page, login with your email address and password, select My Products.
This will take you to all your registered products, serial numbers and download links.