You dont need to add that there that is what the associated platform tab is for. If you add it where you have retroarch wont work for other systems as it will always load the neo geo pocket core.
I think what may be happening here is you were previously using a different emulator maybe, or a Retroarch with a different path, in which case you need to bulk edit the games to use a different emulator, just adding it and putting in the platform wont automatically change the emulator the games use. Click on a game in the platform then use CTRL A to highlight all the games in the platform, right click one and go to edit, this will open the bulk edit wizard. Click next, then in the first dropdown pick emulator, then in the second pich the actual emulator you want to use, finish out the wizard and try loading a game.