Wizards! Where is Lemuria? How do you get there? Great questions!
There are two preceeding quests to complete before you can enter Lemuria. Assuming you have completed the main storyline up through Karamelle, talk to The Old One in the Arcanum to begin them.
Once you've completed the quests, you can then speak to Stallion Quartermane at the Cabal Door which is also in the Arcanum.
