What is Sky?
Tu'Lia or "Sky", as it's commonly referred to, is the endgame area where Zilart Missions 14-17 take place. It is also home to the 5 Sky Gods; Genbu, Seiryu, Suzaku, Byakko, and the "King" Kirin.
How do I get Sky access?
By completing missions 1-13 of the Rise of the Zilart expansion you gain access to Sky.
How do I start the Zilart Missions?
Get Rank 6 in any Nation's storyline and have the Rise of the Zilart expansion installed and registered. You should be able to start the Zilart Missions after that.
What are the Zilart Missions?
The Zilart missions are several quests/BCs/running around aimlessly things you need to do. I will list them and what they are.
ZM1 Cutscene (Enter Norg)
ZM2 Cutscene (Gilgamesh of Norg)
ZM3 Cutscene (Khazam Village Chieftaness)
ZM4 BCNM (Temple of Uggalepih/Den of Rancor. Fight 4 Tonberry NMs)
ZM5 Visit the 7 Cermet Headstones
ZM6 BCNM (Quicksand Caves/Chamber of Oracles. Fight 3 Antican NMs)
ZM7 Cutscene (Chamber of Oracles)
ZM8 BCNM (Upper Delkfutt's Tower/Stellar Fulcrum. Fight Kam'Lanaut)
ZM9 Cutscene (Gilgamesh of Norg)
ZM10 Cutscene (Door at the end of the Hall of the Gods)
ZM11 Cutscene (Gilgamesh of Norg, Maryoh Comyuja of Rabao)
ZM12 Fight (Quicksand Caves. Ancient Vessel ??? spawn NM. Maryoh Comyuja of Rabao)
ZM13 Cutscene (Hall of Gods Door)
Once you touch that door in the Hall of Gods after completeing ZM12, you should have access to Sky.
There ya go.