it saves to the brain age cart. i believe it writes over the brain age save data so you wont be able to save the brain age game if you save sim city on it.
it works if you swap the passcard in slot 1 with a ds game containing save type of 2mbit flash. im playing it using Brain Age: Train Your Brain in Minutes a Day!. just swap it before choosing the game. not sure if all ds games with 2mbit save type work but dont see why it wouldnt.