Got network manager working in Ubuntu LTS 6.06 (Dapper Drake)...
1. Install with Applications->Add/Remove... Internet->Network Manager (or apt-get install network-manager-gnome).
2. Disable your network interfaces with System->Administration->Networking and for each device Properties->Enable this connection (uncheck). Otherwise Network Manager will not have any interfaces to use.
3. Add the applet with System->Preferences->Sessions Startup Programs->Add and type in nm-applet.
4. Log out and log in again to have it all working!
For full instructions see the Ubuntu wiki.
As a side note this install crashed logging out of gnome. First I got a white screen with a working pointer, not response to alt-ctrl-backspace. Going to terminal with alt-ctrl-F1 caused stripes to appear and then the whole OS locked up. Grr