Pokesav.org

It has been confirmed that the Japanese edition of the Nintendo DSi will not work with homebrew slot-1 devices, probably due to a new built-in flashcart protection. Since the DSi also lacks a slot-2, this probably includes the Action Replay DS as well as all current homebrew carts. Consequently, WiFi save backup tools (like Savsender and Rudolph's WiFi Backup Tool) and ARDS codes cannot be used on the DSi hardware.

If the DSi is (or is becoming) the only DS you own, you will not be able to use Pokésav. Until this new protection is hacked or new DSi compatible homebrew/cheat hardware is released, we suggest that you do not let go of your old DS or DS Lite.

Source: GBAtemp