To complete the tank you must attach the makeshift turret.
To do this, you will need to make :
a makeshift barrel. ( duct tape + metal tube x2)
A makeshift base ( duct tape + corrugated iron x2)
Combine these two together with duct tape to make the turret.
Approach the broken tank with the turret selected and press
to attach it and pop your achievement.
Note on new items :
In this dlc there are new items you need for the turret.
Metal tube and iron :
You need a red staff key for both of these. Enter the building in the south west of the compound and you'll see a desk with a star on (Captain America) style). You will usually find one of each in this desk, so you will have to return on another day to obtain two required of each.
Or, if you don't want to wait, use a red staff key to enter the metal workshop. You'll find another desk where you can usually get another one of each.