I am volunteering to help a low-income shelter in San Francisco.  I have donated Lubuntu computers, but the volume control is not appearing on one of the machines.  I followed the instructions on line by doing this:

You will note that Volume Control already is listed on the list of "Currently loaded plugins", but the volume control does not appear on the panel.  Here is a shot of my panel:

I have tried clicking on the "add" button on the right of the Panel Preferences window above, but doing so only puts another instance of "Volume Control" under the list of "Currently loaded plugins" but the icon does not appear in the panel. 

Thanks in advance for trying to help!