Unfortunately in Ubuntu 13.10 there is no tray icon and whitelisting is removed.
The older solutions do not work for me, e.g. I get the message:
No such schema 'com.canonical.Unity.Panel' when I try to use the command:
gsettings set com.canonical.Unity.Panel systray-whitelist "['all']"
How can I get tray icons back (for applications that put an icon in the tray (like TrueCrypt, Pidgin, Aqualung, gnome-system-monitor and so many more) in Unity?
The most annoying thing is that the user loses control over applications which cannot themselves detect that they have no tray icon (e.g.: gnome-system-monitor, TrueCrypt...). At least Aqualung sends a message about being unable to put an icon in the system tray!