here is an example of the script you should use (ignore the lines that have ; at the start, just showing you how to add the weapons and ammo)
copy the script below all this explaination into a text file and save as (without the quotes) "ammocrate" (it doesnt really matter what you call it as long as you remember what you caled it)
then add a ammo crate (doesnt matter which one) and then in the initialisation box type
[this] Exec "ammocrate.sqs"
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
SCRIPT
(copy below this line, add to a txt file and save as ammocrate.sqs or as whatever)
; Get the crate parameter given
_crate = _this Select 0
; Remove the stock items from the crate
ClearMagazineCargo _crate
ClearWeaponCargo _crate
; Add the items to the crate
_crate AddWeaponCargo ["HK", 5]
_crate AddMagazineCargo ["HK", 25]
_crate AddWeaponCargo ["LAWLauncher", 5]
_crate AddMagazineCargo ["LAWLauncher", 25]
_crate AddWeaponCargo ["Binocular", 5]
Exit