I have no idea what directory the Angry Birds resources would be in, I don't own any iOS devices, so don't know anything about the directory layout of them.
Also, as I realised they are Lua scripts, disregard my suggestion of using a Hex Editor. Instead, get a Lua decompiler (just google for one, I have never used one, so don't what one to suggest), and then edit the decompiled levels in a text editor (Notepad or similar would do the job, but I recommend something like Notepad++). On the N900, you don't need to recompile the levels, just copy the decompiled level back to your phone, so I assume its the same on iOS devices.