The drag-n-drop workaround works depending on your theme. That means, if you have "MurrinaStormCloud" as your current theme it works, but if you use "Human" instead it doesn't. So you need to set your theme to "MurrinaStormCloud" and drag the mixer-plugin to the panel instead of clicking on "Add".
See http://ubuntuforums.org/showthread.php?t=375118
I can confirm this bug.
The drag-n-drop workaround works depending on your theme. That means, if you have "MurrinaStormCloud" as your current theme it works, but if you use "Human" instead it doesn't. So you need to set your theme to "MurrinaStormCloud" and drag the mixer-plugin to the panel instead of clicking on "Add". ubuntuforums. org/showthread. php?t=375118
See http://