If you enter an address or location, it should highlight on the map. Then tap on the highlighted item. It should give you a window and one of the items on that window should be Navigate. When you tap on Navigate, it should ask if you are walking or driving. Tap on Driving and it will go to the Navigation app and guide you to your destination.
At least that is how it works on the Desire Z. On the Desire Z there is also a Navigation icon. If you star that, it asks you for a destination and jumps directly to the driving directions.
