Welcome to the Magix user to user forums, so not a contact for any Magix staff.
Assuming you bought the software from Magix you need to contact sales@magix net and ask for the refund. If you have not had the software very long, certainly less than one month, then a refund should be possible. If you bought from a reseller you will need to contact them directly.
However, I have to ask, since there is a Free (basic) version of MM, did you not download and install that before you bought the Premium version, in order to make sure it "worked" for you?
Also, what is the problem of MM on your computer? To simply say it doesn't work really tell us nothing and this forum is visited quite frequently by users who would be more than happy to try and help you get it working properly if you are prepared to give us full detail of your computer and the problem you are seeing.