HaloMods "Classic" Read-only Archive

Okay I moddled the AR now but when i save it in Milkshape it saves it as a .ms3d file and HMT will only accept .obj file.
So how do I get it back into Halo?

I still can't open up the weapon model in Blender or Milkshape.
I can't open any of the sub objects

Okay I just did it and Gmax says "File Open Failed"
IDK If I did something wrong????

Okay how do I take the Assault Rifle Model and put into Gmax then once i'm done remodelling it how do i get it back into Halo PC??