On your Mac, enable photo sharing in iTunes. Don't know why you have to use iTunes - I guess ATV doesn't know how to read the iPhoto library directly. I'm not in front of my Mac, but you can Google the photo sharing menu command if it's not obvious.
On the ATV3, go to the Computer menu/icon, click on your computer name if needed, then go to Photos. Your shared albums should be in there.
From the main Settings menu/icon, you should be able to select one of your albums to use as the slide show.
That said, it's easier to use Photo Stream! Let me know if you need help with that.