Seems to me the #1 question I always had on expansion day was "How the heck do I GET there?" This one's easy:
Go the the Spire of your choice. All spires in the Spire Network hook to the Sundered Frontier now. When you get there immediately apparent to you should be a scattering of city representatives. Go talk to yours.
I put a bit more info, like how to get around in the expansion, on the Sentinel's Fate page.