MEP 18 should run fine on windows 8 as according to the compatibility center, it is compatible. I personally despise Windows 8 as I see it as a OS designed for tablet computers and for me is pretty much useless for a laptop or a desk top without touch screen. My colleague, Johnebaker, has a much different opinion, so I hope he chimes in on this thread. I would hate to taint you to my point of view without you hearing a different one.
If you have the disc version - then the only extra thing you have to do is make sure that MEP MX is patched to the latest version.
Otherwise download the the latest installer by going to the Magix site and clicking My Account top right on the home page.
Login using the account you created when you registered the software then select My Products from the left side of the next screen - here you will find your registered products downloads and serial numbers.