Ok open up the zip file, there should be a dFb_StealthTrans2anf folder when you open it.  Double click on that folder.

Copy UTdFb_StealthTrans2anf.ini into Documents/My Games/Unreal Tournament 3/UTGame/Config/ 

Copy dFb_StealthTrans2anf.u into Documents/My Games/Unreal Tournament 3/UTGame/Published/CookedPC/Script/. If this path doesn't exist then create it. 

Open the game and see if the mutator exists.