Yup it is the positive out. So in between of the charge board positive out and the positive lead of the AGB. Remember that these Diodes are directional so you have to get the polarity right. Not a big deal if you don't, its not gonna short anything, its just going to stop the current from going to the AGB logic board.that does help a bit actually, so just to confirm you've got the diode going from the out on the charge board to the in on the gba board?
its always a good idea to test before your permanently solder.