This should work if you place it in the trigger's init.
_ammo = "USBasicWeaponsBox" createVehicle getMarkerPos "marker1";
Also place down a marker where you want the ammo box to spawn and name the marker "marker1" (no quotations).
As for the classnames ftp://ftp.ofpr.info/armad/docs/OpHandbook%20for%20Unsung%20ArmA%202%20-%20Release%202.pdf
At the bottom of that pdf is the list of most of the classnames, except I couldn't find the classnames for the ammo box's. But there is a way to get the classnames. Place the box you want down and put this in its init
hint format ["%1",typeOf this]
then preview the mission and in the top right it will show you the classname and just replace the USBasicWeaponsBox with the classname of the box you want. For example the us army armory in unsung is uns_AmmoBoxUS_army so the code would be
"uns_AmmoBoxUS_army" createVehicle getMarkerPos "marker1";