This is a sound pack that mutes the sound played by the group finder when someone applies to your group.
To install it, just extract the downloaded zip and copy/paste the Sound folder into the main WoW directory. If it asks if you want to merge folders, click Yes. Do not install via the MMOUI Minion or Curse Client as they will try to install this as an AddOn (which won't work).
I've created a visual installation tutorial which you can view here.