The dried tastes different from fresh. In the dried there's a sweet and a bitter variety, so watch out. The sweet tastes better than the bitter.
I prefer fresh. Just make sure not to cut into the white pith. It is very bitter and not in a good way. Just grate the orange material, nothing else.
Add 5 minutes before flame-out, but may still be too long. Maybe better during a hop stand (150-170°F). I think you can "dry-hop" with fresh zest too, but I haven't tried that yet.
I've also boiled some for 1/2 hour to obtain a different, deeper flavor profile, but I wouldn't recommend it in general.
I've read using coriander seeds (Indian store) actually creates more orange flavor than zest does. Do a search.