Kekropus has said that in order to go meet N, you need to kill Tiberias in Nornil's Garden and use his key to open the door to the Large Cavern. There are three gates, and the codes for the gates are the same as the codes you used to communicate with N. For the Icon of the Past, the code is CRTR. For the Icon of the Present, it is CNCL. For the Icon of the Future, it is CHAOS.