Hi AdiumX. Haven't had your problem, but I can at least give you some things to consider, assuming you haven't already solved it!
Which version of 10.6 are you running?
Myst V has performed flawlessly for me on all versions of 10.6 from 10.6.3 to 10.6.8.
Have you installed Rosetta? Without it, Myst V won't start (nor will any other PowerPC-only program you have installed)!
OS X 10.6 doesn't install Rosetta by default. However, if 10.6 is functioning properly, it should prompt you to install Rosetta when you try to launch a PowerPC-only program, if it isn't installed already.
Assuming it is installed, here's a couple of simple tests you can try.
Open your home/Library/Application Support folder and delete the Myst V folder located there, then try relaunching the game.
If unsuccessful, log in as a different user and try launching the game from there (you may need to reinstall Myst V to perform this test).
If neither of these suggestions help you, then you're looking at a potential Mac OS X reinstall (particularly if you're running 10.6.7, or have updated to 10.6.8 from 10.6.7).
By the way, whenever your Mac misbehaves, it's always a good idea to restart it before you try anything else!