This is not really an AddOn, but a zipfile with a couple of soundfiles that will replace the sound of your gun by star wars blaster sounds. It also includes the humming sounds that lightsabres make, but without these it would sound silly.
Unzip the zipfile.
You should now have a "Sound" folder.
Move the sound folder into your "Data" folder under your "world of warcraft" folder.
For example, on windows if you installed world of warcraft here: C:\Games\World Of Warcraft\Data
If you see the following file under explorer:
C:\Games\World Of Warcraft\Data\Sound\Item\Weapons\GunFire01.wav
then you installed correctly.
Go into above mentioned directory, and delete all of the Gunfire/Gunload files.