. . . . need a new mp4 key under windows 10 . . . .
Not a lot to go on!
More information is needed:
Your computer specification?
Movie Edit Pro version? If 2015 or 2016 see note 1 below.
Have you upgraded to Windows 10 from 7 or 8 and now the the MainConcept codec will not reactivate? see note 2 below
Otherwise give more information about what has caused the need for a new mp4 key.
Note 1 - by default both versions will use the Intel HD codecs, this requires a suitable Intel processor. If you have an AMD processor the MainConcept codec has to be purchased.
Note 2 - you will need to uninstall and reinstall MEP following this procedure