There were a few issues with the overlay installer package shipped with Mumble 1.2.3 on Mac OS X.
What it boils down to is that on some configurations, the Mumble app cannot verify the authenticity
of the overlay installer and will refuse to install it. This is because the overlay installer is digitally signed,
but not all the information needed to perform a complete verification exists on all systems.
If you are seeing this issue, you can use the following Mac OS X installer file to properly install the overlay: http://mumble.hive.no/snapshot/MumbleOverlay-1.2.3.pkg
For a more thorough and technical explanation, please see this bug:https://sourceforge.net/tracker/?func=d ... tid=768005
Enjoy the overlay in Mumble on Mac OS X,