Make sure you have the titleID properly set up. It should be one of the two I posted in my reply to this thread earlier.
It could also be your text editor. I use Windows, so the default Notepad does the job. I'm not sure if the Mac OS text editor adds anything to the file or if it's stored in Unicode or something. It could probably throw the game into a crash, but I'm not really sure.
If it's not that, then you can try either using GodMode9 to make a CIA of your cart and install it (like everyone else suggested already), or try a different Japanese cart of the same Animal Crossing game in case it went bad. If you do dump your cart via GodMode9, use JK Save Manager to copy your save from the cart to your CIA dump after you've installed it.
If all else fails, maybe a Japanese emunand might fix it.