Check all the steps below:

You have to extract the addon. Create a new folder with a name without capital letters, spaces and special characters. You have to drag the lua folder that is in the RAR or ZIP file that you have downloaded, into the folder you just created. Put the folder you just created in the addon folder of your game or server and restart your game or server.

If it still doesn't work, check that your keys are properly configured.

This addon to only was tested for the official M9K weapons.

If this still doesn't work, the problem doesn't come from my addon but from your game. Try to clear the cache.