To unlock the MG 42 LMG in Call of Duty: WW2 you have to level up the armored division all the way to the end of level 4. The way that you level up the armored division is to use it in one of your classes during matches. Then enter the headquarters and go see the division prestige guy. Select the armored division and proceed with the prestige. This unlocks the MG 42 LMG no matter what your soldier level is..
As found on Youtube