The only version of Universal Control that is supported on Mac OS X 10.5 is Universal Control 1.2.
You may have installed a newer version of Universal Control that does not support this old version of OS X.
Please review this article on how to remove Universal Control from your system:
Then once it's removed, you'll need to reinstall Universal Control 1.2 found at the very bottom of this page:
If you are still having this issue, then there may be a problem with the 1394 ports either on your Mac or on the StudioLive 16.4.2
1394 / FireWire is NOT a hot plug technology like USB is. You must power down both the mixer and the computer before connecting or disconnecting the 1394 cable.
Try plugging a 1394 Hard Drive into the computer to verify the 1394 port is working, and/or plug the 16.4.2 into another computer to verify that the 1394 on it is working. If the mixer is still not recognized then you'll need to contact us to see about getting the unit repaired as the 1394 port may no longer be operational. You can contact us at http://support.presonus.com