Into the weapons list dropdown itself when you try and add weapons to the box? It should already have every single weapon you have installed in your game, excluding a few blacklisted ones and weapons that have no world model.

If you just mean adding weapons to the random box in general, go to the Map Settings tool with Advanced Mode on and you'll get a tab that allows you to decide exactly what weapons you want in the box and how rare they are compared to each other.

Thanks,really helped.

For some reason, even though I go to the settings and edit what weapons are in the box, they still wont update. I submit my changes, and when opening the box, it will scroll through the added weapons, but never give them. I even changed it so there was only one weapon in the box, and it still ended up giving what I assume is a weapon on the blacklist. I can't add anything to the box for some reason. Please help!

Nevermind! I finally found something that told me to change the map weapon list