eating whenever you want and how much you want is sort of ok as long as it's primal. part of the idea is that it would be hard to over eat on the right foods because you would simply be too full. that's not exactly right, because you can still eat way too many calories and you will gain weight. obviously, loading up on carbs (potatoes, some winter squash types, even nuts and seeds) will also cause weight gain.
just eat when you are hungry--real hunger, not psychological or it's-time-for-dinner hunger--and you'll do just fine.