You have to finish all the Driftwood quests, helping Driftwood expand. I believe the last one is delivering a letter to a carpenter then leave and re-enter the region and there'll be a bridge made of wood to the platform.
Sorry guys, but no. There are three parts to finishing Driftwood.
Finding and bringing the carpenter TO Driftwood is only step two.
After that, in Bowerstone Market, there is a quest called "Island Paradise".
There is a rich gambler wanting to start fresh in Driftwood, and he needs you to escort him there.
Once you have done that, go back to playing Fable as you would normally; When you see an achievement pop up on your screen saying "Island Paradise", you may return, and there will be a plank for you to reach it with.
Hmm, well seeing as I haven't done the Island Paradise quest and I am able to go across to the island where the target is, I would have to say something is up there. I was able to cross to it and do the magic, gun target and sword hitting until it's on a rock and has a sword target on it. I cannot reach the target as its slightly above me. I have tried to go above and find a spot to vault, to no avail. I am on the island with the ruins, anyone know how to get to it? *edit* I have done all the other quests in Driftwood, just not the Debt quest.