To get to the now-opened Skyarrow Bridge, Fly to Castelia City and head east along the street from the Pokemon Center, to the gate at the end. Head through the gate and you'll be at the foot of the bridge! Head along the bridge. There's not a whole lot to do here.
The only thing of note is the man around halfway across the bridge selling Fresh Water for $300 each. Overpriced, but if you feel the need to buy one, do so. If you speak to him and decline, you'll receive the hard sell. If so, simply decline again if you don't want to buy and he'll stop bothering you.
If you do cave in and buy the Fresh Water, there's a dehydrated man further on who's in desperate need of it. You'll also find him gagging for water elsewhere in Unova's bridges. Once he's had his fill, he'll give you a bunch of MooMoo Milk as a reward.
Anyway, continue until you reach the gate at the end. Enter it and move through into Pinwheel Forest on the other side.