Download http://download.dfbgaming.com:9080/UT3/dFb_StealthTrans2anf.zip 

If you don't how to open and extract a zip file: http://www.bleepingcomputer.com/tutorials/tutorial105.html

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

Both the script and ini file are in this folder (dFb_StealthTrans2anf)

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.