I have Andorid maps 5.0.0 running on a Nexus S but I can't seem to get 3D maps. I have tried London and New York. Not sure if I am doing something stupoid or what.
3D maps includes 3D models of some cities. London isn't included, yet, but New York is, as are Cardiff and Birmingham on the UK. You don't need to do anything to view them other than select the map layer, zoom right in, and then swipe down with 2 fingers to angle the view.
(the 3D models appear if you're looking straight down, but it misses the point, somewhat...)
Make sure satellite layer is off, and swipe the screen down with 2 fingers.