For photos you need to go to iTunes - advanced - choose photos to share. You can access via computers on ATV.
Videos would need to be imported into iTunes and then can be selected via the computers section.
I ahve turned on home sharing on my advance itunes. When I go to ATV and select computer it say 'to access itunes on home sharing turn on home sharing using apple id (then my id) I have done this and the ATV comes up with this menu everytime I selct computer from my ATV