Nope, not mission editor. That's for making terrain.
To make models: use a modeling program, such as Milkshape 3D (which costs money) or Blender (which is pretty complicated).
For the script: use a program like notepad. Any text editor will do. Usually, you can just edit other scripts to make your weapon the way you want.
Rock Band, I'd suggest that you stay out of these topics unless you know what you're doing.