Just head over to Software Center and search for Gnome Shell. You'll probably want gnome-tweak-tool as well to change the theme. Unfortunately, the default Gnome 3 desktop doesn't have a theme setting tool. Ubuntu's Ambiance and Radiance also do not work for Gnome Shell.
|gnome-tweak-tool themes settings|
Once installed, you'll want to use gnome-tweak-tool (it shows up as Advanced Settings) to change both the GTK+ theme and Window theme to Adwaita. Then, logout and login again (choosing Gnome as your desktop, not the default Ubuntu). For some reason, Unity changes the theme immediately but Gnome Shell doesn't until your next session login.
If you're intent on taking Gnome Shell for a spin then it's probably better to do so under Oneiric. Gnome Shell isn't without issues (Oneiric is still under development) but it's much less problematic than it is on Ubuntu 11.04 where you need to practically gut the system to get Gnome Shell.