Assuming you have The Shadows of Luclin expansion pack, you can go to the spires in North Karana near the bridge to South Karana and talk to the Nexus Scion. Follow the dialogue and you will get a nodrop item that you hang onto. Wait (for up to 15 minutes) and you will be transported to the Nexus on Luclin.
Alternately, if you have PoP, go to the Plane of Knowledge via any book, and then enter the Nexus or Shar Vahl via the appropriate stone.