It is a little bit involved, read this thread for hints:-
1). You recreate the folder structure of the guns prop in your own Addon.
2). Create your prop in a software like Milkshape3d (or other)
- it needs to be placed in the correct position relative to the skeleton so I normally import the MS skeleton into my model and line the gun up.
- attach the gun to the correct bone in the skeleton. Then
- Export it in CAL3D format
3). Place the mesh, material and texture files in the correct folders as above (.cmf and .crf go in the prop lever textures go in a folder below that called "Textures".
MOST IMPORTANT! Delete the skeleton file you exported (this is the .csf file) If you don't it will conflict with the Moviestorm skeleton and crash MS every time.
4). Publish your Mod.
Then the new gun will appear as a variation in the menu when you choose "Use Prop".
For more details check out Tutorial no 5 Held Props" under Advanced Modding on this page
P.S. If anyone wants the gun addon I did (see here) PM me and I will send you the addon for 100 MS points.