The Panel is the taskbar at the top of the Unity desktop interface in Ubuntu. Many GUI applications that persist in the background, like those with notifications, like to add an icon to the system tray in the Panel. However, you might find that a newly installed application does not appear in the Panel, even though it is supposed to.
Only applications in a pre-defined whitelist are allowed to add their icon to the system tray in the Panel. These include the default applications that ship with Ubuntu and a few others. All other applications have to be manually whitelisted if their icon is to be allowed to appear in the Panel. To do this:
$ sudo apt-get install dconf-tools
Open DConf Editor from the Dash or
dconf-editor from the shell.
Go to Desktop > Unity > Panel. Click on the value field for the name
Add the binary filename of the application you want to whitelist. Add this name enclosed in single quotes to the list. Close DConf Editor.
Start your application again. Its icon should now appear in the Panel.
Tried with: dconf-tools 0.12.0 and Ubuntu 12.04 LTS