One week ago, I upgraded my Ubuntu 10.04 to 12.04 LTS. After the upgrade finished, I could not see the network manager icon in the tray. The first thing I tried was to install the gnome-network-manager in Ubuntu Software Center. To my surprise it was not possible.
After searching for a while and trying some suggestions that were written in the forums, I found this link to a website called ubuntuupdates.org:
Here, I clicked on the button called "APT INSTALL" under Download "network-manager-gnome", selected Ubuntu Software Center in the Launch Application window that pops up and proceeded with the usual installation steps.
To test whether the network manager is installed properly you could try to run nm-applet in terminal.
The final step is to add the nm-applet to Startup Applications. Open the Dash Home, enter Startup Applications and add the applet there. The network manager will load every time you start Ubuntu.