First, you have to be at the right level in the game. I think it's 80, but may be 70. Then you have to talk to the guard at the portal, who will give you a quest to find some 'gillyweed'. Once you have done that, you will be allowed to go into the zone.

I don't understand why. You just have to walk through. Once you have the gillyweed, just walk through, ignoring any message. It may still say you need something, but if you click slightly to the left or right on the gate posts, you can go through.