It's just a short boat ride from Domnann. It's been a while since I was there, but I *think* if you go out the entrance of Domnann Grove and turn left, then just keep going until you hit the water, the dock isn't far.
But if you're a relative newbie, you really don't want to go to Atlantis. I went there as a 20-something, and I won't go back until I'm at least level 40. Getting scragged by deep-purple mobs every few minutes is NOT my idea of a good time.