The registration code that you enter is only saved when the game is saved. So if you never exit the game (such as resetting instead of exiting to the Palm launcher), the registration code won't be stored.

As I mentioned in my earlier post above, that problem is also frequently caused by an incompatible or corrupted savegame file. There are several sets of circumstances that can cause the problem, including a savegame file corrupted by a crash, or installing an updated version of Village Sim without completely deleting the previous version first. The simple fix is to completely uninstall the game from both internal memory and the expansion card (if installed) and reinstall the game. Once you start a new game on the clean installation of the game, register it, and save it (by exiting to the launcher or another application), you shouldn't have further problems.