Hi, welcome to the Magix forums.

First, you need to be aware the MMM MX is now several versions old so may have problems running on a modern operating system.

Second, as you have discovered, you need a serial number in order to install and use the product. Where and from whom did you buy the "installer"? (I assume it is on a CD). I think you need to ask the seller about the missing number and if he/she fails to supply it you should ask for a refund and return the disc. Hopefully, wherever you are in the world, there is legislation which will assist you if the seller fails to respond.

Please let us know how you get on.

Jeff