Default how to modify an system app after get ROOT?

RC19.

How to replace app like AlarmClock.apk?

I want to replace the system one to a localized AlarmClock.

I did the replace, but runtime error occurd.

Why? Is it need the sign or something? Or is my source code too old?

Im use TC3 code to compile the AlarmClock app which works fine in my emulator.