1. there is no "editing program" for addons, except stuff like Notepad++ for coding, and modeling programs like blender.
2. If you look around in general mod help, there's a general blender help thread(skim through some pages) and a guide to exporting in blender for models. There is also a Big list of tutorials that even teaches you how to make weapon models for blender.
3. there's also script tutorials, and scripting takes a while to learn.