Examine closely the command reference in the Editors Depot and make sure you are getting the syntax right. There is also a weapons and ammo listing in the tutorial section so you can check if you have the right names, though obviously it doesn't cover addons. Try and make it work with game weapons first, then at least you know you have the correct command.
One tip: always add a magazine before you add the weapon, that way the weapon will be loaded at the start of the mission.