10.6.1 + m257 should work out for you...

Ensure you have the latest Decklink drivers and that you can see an image via blackmagic software outside of catalyst
If not - Ask Richard for an update - try it without upgrading the OS, unless Richard says otherwise.

are you running in 32bit mode?
What version MacPro do you have? (2010?)

PROBLEM:
2010 MacPros Boot in to 64Bit
- By default all new 2010 MacPros boot in to 64bit mode. However, most apple applications do not support 64bit mode - including quicktime. Quicktime is still a 32bit application. Active Silicon Drivers (and many others) are currently only 32bit.
SOLUTION:
Force the Mac to boot in 32bit mode.
To select the 32-bit kernel for the current startup disk, use the following command in Terminal:

sudo systemsetup -setkernelbootarchitecture i386

This will allow the Mac to boot in to 32bit mode permanently
(an alternative command is available to boot in to 64bit mode)



NOTE:
These issues do not affect users using Mac OS 10.5.8 Leopard.

Down-grading the MacOS is only an option with a MacPro version earlier than 2010.
Any current 2010 versions available from apple cannot be downgraded to previous software version.


S