If you are trying to play a backup (USB or disc), you only need an up-to-date loader and waninkoko's or hermes's cIOS.
However, if you are talking about a retail copy, you need to have IOS53 installed. You can download the current version of IOS53 using NUSD, pack it into a wad using NUSD, and install using wad manager or one of the mods (i.e. Yet Another Wad Manager Mod).
EDIT: as Blue-K said - put a little effort into searching next time.