The best way to do this is to copy a map that already has guns. Here is a tutorial:
How to make your own razanak levelsThen, you just clear the map with the /newmap command, and voila, you have guns
If you want to add custom guns, look at the files in intensityengine/packages/library/1_3 and look for the guns, projectiles, firing js files, and they'll show you how to make your own gun (just copy one of the existing guns in one of those files, and paste it into your map.js file, and tweak it to your liking)
If you want more help, pop on the irc channel