You can't do it. You can only copy a save file to the same account on a different unit.
So if I have two PS3s, I can copy my save/garage from xViLLaiNx12 and put it on my second PS3 with the screen name xViLLaiNx12. If you put it under a different PSN name, it won't work.
BTW: You pop in the USB drive to the PS3, go SAVEDATA and copy that onto the stick. It's as easy as that.