so you have enterred the MPEG 4 activation code where it is asked for, then when you choose export, nothing happens. If not, check under Help, activate additional functions and see if MPEG4 appears; i.e. you can choose it.
If you can choose it, follow the procedure and copy and paste the code in which you were sent.
This is an old program, then MPEG handling, 2 and 4 is limited in the classic version. If you are trying to export AVC-HD, then it won't necessarily work.
MY suggestion - Get in touch with MAGIX and get the latest version MEP 17 Plus, and if you ask nicely, I'm sure they'll offset what you spent on the encoder. Even if you get it working, the handling of these files has improved immensely over recent versions, that you're far better off with the latest.