On the right side of the flight master screen are different territory options. There are 3, one for each clashing faction. Dwarves versus greenskins, elves versus dark elves and chaos versus empire. I believe Altdorf is under chaos versus empire, but it's 8am and I'm only half functioning, so I could be wrong!
Click on those links and you'll see a big green circle that says Altdorf. Pay the nice man some money, hop on a gyrocopter and you'll find yourself in the massive city. It's good to go explore and reap some discovery xp ; )
Svaia (cleric), Vendolyn (mage) some random beta server
Vendolyn (warden) - Ananara (mystic) - Patreska (monk) Unrest server