I can't say for sure that this is how, but this is how I remember it (not on a computer with US2 installed)
1. Open a sim
2. Click "View" towards the bottom left corner
3. Second from the bottom you should see something along the lines of "Habitable Zone"...? I forget
4. Choose an option for how you want to view the habitable zone (billboard, flat, 3 dimensional)
Hope this helps!