To show the current whitelisted programs, run
gsettings get com.canonical.Unity.Panel systray-whitelistTo add a new program; for example, Tasque, to the current whitelist, run
gsettings set com.canonical.Unity.Panel systray-whitelist "['JavaEmbeddedFrame', 'Mumble', 'Wine', 'Skype', 'hp-systray', 'scp-dbus-service', 'Tasque']"Alternatively, we can do this through a GUI using a tool called "dconf-editor". It's part of the "dconf-tools" package. In dconf-editor, go to
desktop -> unity -> paneland select "systray-whitelist" on the right panel.
After change, strangely doing a relogin didn't seem to take this into effect for me. I had to reboot the PC. I didn't try just killing X Window though.
Refs
http://www.omgubuntu.co.uk/2011/03/how-to-hide-or-show-app-tray-applets-in-ubuntu-11-04/
http://ubuntu.sun.ac.za/wiki/index.php/Show_tray_icons_in_unity