Well firstly, do you mean that you copy it into saved game data utility fine, but when you go into split screen and try load it, it isn't there?
If so, that's because its in a different format. Say you just got the general one off from a tut, it would most likely be NTSC (American) unless otherwise specified, if you are trying to use a PAL Ps3 (Europe/Australia/New Zealand, etc) to open it, it wouldn't work.
You HAVE to get a save file which corresponds to your region.
Also if your doing the Prestige hack, make sure you're doing the bypass!