In order to level beyond 65 and access the new zones, dungeons, raids, and more in Prophecy of Ahnket you must actually own the expansion. You can find more about the how to purchase and the features of Prophecy of Ahnket here.
Once you own the expansion you can get to the new zones by talking to any Porticulum Master and selecting the 'Celestial Lands' location. From there you can start your adventures for level 65 and beyond!