Extract ZIP file to "Runes of Magic\Interface\AddOns\" folder. If you don't have the Interface and AddOns folder, create them.

Verify that you have the files in the proper place, which should be:

Runes of Magic\Interface\AddOns\WarningOverride\WarningOver ride.lua
Runes of Magic\Interface\AddOns\WarningOverride\WarningOver ride.toc
Runes of Magic\Interface\AddOns\WarningOverride\WarningOver ride.xml

Also, verify that when you load the game, you see the message:

Warning Override addon loaded. /warningmsg command available.