There seems to be a problem on booting homebrew bigger than 2MB.
But here the easy solution for this Problem:
Get
DSision or
Flashcart OS
These 2 homebrews are able to boot homebrew on the N-Card (never saw this working for the N-Card before)
Now Start DSision/FlashcartOS and browse to the homebrew (Example: Tower Defence) and start it.
And... oh wonder, it should work!!