accessing a file using [NSBundle mainBundle] pathForResource: ofType:inDirectory:

I was also having the same problem.
The Solution i found is ( in xcode 4.x):

Go to : Target -> “Build Phases” -> “copy bundle Resources”
Then add that particular file here.
If that file is already added , delete it and add it again.

clean the project and RUN. It works. 🙂