How to set MacPro to sync an incoming spdif signal?

I have a little problem that I do not quite understand. Perhaps you do.

My setup:
I digitize an analog signal via M-Audio's Firewire 410 and output the digital signal via optical to my MacPro's digital in. I record in 24/96 format and have the MacPro's MIDI set up accordingly, also everything is finely set up with my Amadeus Pro recording software. I control the M-Audio 410 through a separate Powerbook that's connected to the 410's via firewire.

The M-Audio 410 control panel has the option to set the sync source (clock) to either internal or external. M-Audio's manual suggests to use the external sync source setting when recording from the SPDIF output (that's what is connected to my MacPro's digital in). However that does not work. The MIDI Setup panel on the MacPro shows a sync-lock only with sync source set to internal in M-Audio 410's control panel.

Is there some way to have my MacPro sync the digital input signal or do I have a wrong view on this issue?

Any informed reply is highly welcome, thanks.