DivxDecoder.dll is missing?

i tried downloading a WoW arena tournament and i did everything the website told me to do. i have a shortcut for the WoW and i click it and it says "the program can't start because DivxDecoder.dll is missing from your computer. try reinstalling the program to fix the problem. i would really rather not download the file again because it took a few hours and i only have 60 gigabytes per moonth and it took 16 to download. anything else i can do???