Install:

Extract contents in rar to a folder

Weapon.xml
//////////
Drag common folder into this directory C:\Program Files (x86)\Rockstar Games\Grand Theft Auto IV 

or where ever Gta IV is installed.





Weapon Models
/////////////
Drag weapons.img folder into this directory C:\Program Files (x86)\Rockstar Games\Grand Theft Auto IV\pc\models\cdimages

or where ever Gta IV is installed.



Sound FX
////////
Open Spark IV select GTAIV at top find the folder pc, next locate audio, then locate Sfx, after clicking Sfx locate resident.rpf

double click it, then click RESIDENT go to import at top and select WEAPONS.ivaud from the rar folder.