You can put your phone into Car Mode by either putting it in a compatible car dock or by using the default Google Car Home icon (the Steering Wheel).
After pressing the steering wheel, it should ask you which car home you would like to run (either stock or Car Home Ultra).
If you don’t get the option, you can reset the defaults for Car Mode by doing the following:
1) From the main home screen press menu –> settings –> manage applications
2) Pick Car Home (the one from Google with the steering wheel icon)
3) now press Clear Defaults.