There's a difference of five versions of MMM (there was no MM-13) and they are made for two vastly different operating systems. It's not likely you'll be able to use your MM-12 projects in MM-MX. But, I could be wrong, and someone may chime in and set the record straight.
But, I'm curious...why is that even necessary? Odds are the vast majority of your MM-12 projects are finished, so what is the need to run them in MM-MX? Just be happy with them the way they are and move on. Otherwise, you can always recreate them in MM-MX.
Procyon has already answered but I would like to add further.
You asked
"How do I import mmm.files from musicmaker 12 and play them in music maker mx?"
When we create project in Music Maker, the project we save could be based on, Soundpool in wave/ogg or tak formats and different Synthesizers. Then the soundpool/synth we use their file path is also associated in the projects. Just by a slight difference in file path when we open project we will see error, File not found or path changed etc.
To use your MMM 12 Project you need to do some work:
1. Copy your BASIC (soundpool)folder from MMM 12 to computer or on external drive.
2. Copy synth folder from MMM 12 to computer or on external drive. (then add)
3. Remember where you have saved your old projects, separate hard drive?
Note in PDF page 121 of MM-MX it states:
------------------------------------------
"Note for users of older Music Maker versions:
This version of MAGIX Music Maker has replaced older technically outmoded
synthesizers with newer, significantly improved solutions. Other synths were
replaced by newer, non downward-compatible versions.
Arrangements created using older versions of MAGIX Music Maker may not
function properly MAGIX Music Maker MX for this reason. MAGIX provides the
removed synthesizers as an extra download.
To download... http://rdir.magix.net/?page=DEPBU7N8R5E2"
Now open your MM-MX and try to create new project with the soundpool/synth from MMM 12, if it works and you can save the project please save it in the place where your old projects are already saved for testing purpose.
Now try to open the old saved project ,if you can good, surely you will see files not found, because files paths are missing you will have to browse manually and add.(painful task) and re-save the projects.
I do got MMM 10,11 and 12 Deluxe version also MMM 16 and 16 Premium version. In past I have tried this kind of things and can understand what you are up to but that is a painful task.
Note: As Procyon has mentioned about two different operating systems. Therefore, you may not use older projects on MM-MX then what you should do?
If you still got your older computer with MMM 12 on, open that and take a look how did you make the project then apply same method in MM-MX project using MMM 12 soundpools.
Hope the above information might be useful to you, if not then sorry in advance.