Sakura has it's own loader build into the interface. you don't need to use gba exploader to load your games on a M3. You load a game like any DS game.
Reason why you can't use gba exploader on your M3 slot1 to load gba:
Roudolph banned all slot1 M3 products from using the gba exploader after they sent him a C&D.
There is a patch to remove the ban, but it is something your currently do not need to use.
There are several people with the can't load gba games issues. The answer to those was to have the M3 GEP (GBA Expansion Pack) in the DS and charge your DS overnight. Seems the battery in the M3 GEP is slightly rechargable.