1. Navigate to your cod4 folder. (For me it would be C:\Program Files (x86)\Steam\steamapps\common\call of duty 4)
2. Open up the mods folder.
3. Open up the mod folder you wish to add the weapons too
4. Once that is done open up the mod's .iwd file in winrar or 7zip(Example: modwarfare.iwd)
5. Open up the weapons folder
6. Open the mp folder
7. Add the new guns into there.
Hope this helped.