You have to download and install the H8000 Firewire Driver for mac in order to use the audio over FireWire capabilities with your MacPro.
The H8000 routings allow you to send multiple channels of audio from the computer to theH8000 either processed or straight through.
There's a routing utility for PC that makes configuring the routings easy. You can configure them from the front panel, in which case I recommend using the routing presets as starting points. The most commonly used routings are in presets accessible by pressing and holding the Program button.
As for your clock slipping, I would suspect the cables not locking as a problem.