i dont like spoon feeding ......but your having problems and its obvious your not going to work it out on your own so heres a tutorial.
Blahhh = createvehicle ["USBasicAmmunitionBox", position player, [], 0, "NONE"];
Blahhh addweaponcargo ["M14_EP1", 99]
Blahhh addweaponcargo ["M4A1", 99];
it works like this.
Blahhh << gets the object linked to it
Blahhh ( is now the box ) addweaponcargo ( adds the next string defined in bi's wonderfully fantastic 'config' classes to the box ) "M14_EP1" , then the amount to add to the box ( 99 ).
check for the mistake ( if someone posts how to fix the error then they are not helping you they will be hindering you but whatever ).
good luck.